Source file src/go/parser/parser.go

Documentation: go/parser

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package parser implements a parser for Go source files. Input may be
     6  // provided in a variety of forms (see the various Parse* functions); the
     7  // output is an abstract syntax tree (AST) representing the Go source. The
     8  // parser is invoked through one of the Parse* functions.
     9  //
    10  // The parser accepts a larger language than is syntactically permitted by
    11  // the Go spec, for simplicity, and for improved robustness in the presence
    12  // of syntax errors. For instance, in method declarations, the receiver is
    13  // treated like an ordinary parameter list and thus may contain multiple
    14  // entries where the spec permits exactly one. Consequently, the corresponding
    15  // field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.
    16  package parser
    17  
    18  import (
    19  	"fmt"
    20  	"go/ast"
    21  	"go/build/constraint"
    22  	"go/internal/typeparams"
    23  	"go/scanner"
    24  	"go/token"
    25  	"strings"
    26  )
    27  
    28  // The parser structure holds the parser's internal state.
    29  type parser struct {
    30  	file    *token.File
    31  	errors  scanner.ErrorList
    32  	scanner scanner.Scanner
    33  
    34  	// Tracing/debugging
    35  	mode   Mode // parsing mode
    36  	trace  bool // == (mode&Trace != 0)
    37  	indent int  // indentation used for tracing output
    38  
    39  	// Comments
    40  	comments    []*ast.CommentGroup
    41  	leadComment *ast.CommentGroup // last lead comment
    42  	lineComment *ast.CommentGroup // last line comment
    43  	top         bool              // in top of file (before package clause)
    44  	goVersion   string            // minimum Go version found in //go:build comment
    45  
    46  	// Next token
    47  	pos token.Pos   // token position
    48  	tok token.Token // one token look-ahead
    49  	lit string      // token literal
    50  
    51  	// Error recovery
    52  	// (used to limit the number of calls to parser.advance
    53  	// w/o making scanning progress - avoids potential endless
    54  	// loops across multiple parser functions during error recovery)
    55  	syncPos token.Pos // last synchronization position
    56  	syncCnt int       // number of parser.advance calls without progress
    57  
    58  	// Non-syntactic parser control
    59  	exprLev int  // < 0: in control clause, >= 0: in expression
    60  	inRhs   bool // if set, the parser is parsing a rhs expression
    61  
    62  	imports []*ast.ImportSpec // list of imports
    63  
    64  	// nestLev is used to track and limit the recursion depth
    65  	// during parsing.
    66  	nestLev int
    67  }
    68  
    69  func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
    70  	p.file = fset.AddFile(filename, -1, len(src))
    71  	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
    72  	p.scanner.Init(p.file, src, eh, scanner.ScanComments)
    73  
    74  	p.top = true
    75  	p.mode = mode
    76  	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
    77  	p.next()
    78  }
    79  
    80  // ----------------------------------------------------------------------------
    81  // Parsing support
    82  
    83  func (p *parser) printTrace(a ...any) {
    84  	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
    85  	const n = len(dots)
    86  	pos := p.file.Position(p.pos)
    87  	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
    88  	i := 2 * p.indent
    89  	for i > n {
    90  		fmt.Print(dots)
    91  		i -= n
    92  	}
    93  	// i <= n
    94  	fmt.Print(dots[0:i])
    95  	fmt.Println(a...)
    96  }
    97  
    98  func trace(p *parser, msg string) *parser {
    99  	p.printTrace(msg, "(")
   100  	p.indent++
   101  	return p
   102  }
   103  
   104  // Usage pattern: defer un(trace(p, "..."))
   105  func un(p *parser) {
   106  	p.indent--
   107  	p.printTrace(")")
   108  }
   109  
   110  // maxNestLev is the deepest we're willing to recurse during parsing
   111  const maxNestLev int = 1e5
   112  
   113  func incNestLev(p *parser) *parser {
   114  	p.nestLev++
   115  	if p.nestLev > maxNestLev {
   116  		p.error(p.pos, "exceeded max nesting depth")
   117  		panic(bailout{})
   118  	}
   119  	return p
   120  }
   121  
   122  // decNestLev is used to track nesting depth during parsing to prevent stack exhaustion.
   123  // It is used along with incNestLev in a similar fashion to how un and trace are used.
   124  func decNestLev(p *parser) {
   125  	p.nestLev--
   126  }
   127  
   128  // Advance to the next token.
   129  func (p *parser) next0() {
   130  	// Because of one-token look-ahead, print the previous token
   131  	// when tracing as it provides a more readable output. The
   132  	// very first token (!p.pos.IsValid()) is not initialized
   133  	// (it is token.ILLEGAL), so don't print it.
   134  	if p.trace && p.pos.IsValid() {
   135  		s := p.tok.String()
   136  		switch {
   137  		case p.tok.IsLiteral():
   138  			p.printTrace(s, p.lit)
   139  		case p.tok.IsOperator(), p.tok.IsKeyword():
   140  			p.printTrace("\"" + s + "\"")
   141  		default:
   142  			p.printTrace(s)
   143  		}
   144  	}
   145  
   146  	for {
   147  		p.pos, p.tok, p.lit = p.scanner.Scan()
   148  		if p.tok == token.COMMENT {
   149  			if p.top && strings.HasPrefix(p.lit, "//go:build") {
   150  				if x, err := constraint.Parse(p.lit); err == nil {
   151  					p.goVersion = constraint.GoVersion(x)
   152  				}
   153  			}
   154  			if p.mode&ParseComments == 0 {
   155  				continue
   156  			}
   157  		} else {
   158  			// Found a non-comment; top of file is over.
   159  			p.top = false
   160  		}
   161  		break
   162  	}
   163  }
   164  
   165  // Consume a comment and return it and the line on which it ends.
   166  func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
   167  	// /*-style comments may end on a different line than where they start.
   168  	// Scan the comment for '\n' chars and adjust endline accordingly.
   169  	endline = p.file.Line(p.pos)
   170  	if p.lit[1] == '*' {
   171  		// don't use range here - no need to decode Unicode code points
   172  		for i := 0; i < len(p.lit); i++ {
   173  			if p.lit[i] == '\n' {
   174  				endline++
   175  			}
   176  		}
   177  	}
   178  
   179  	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
   180  	p.next0()
   181  
   182  	return
   183  }
   184  
   185  // Consume a group of adjacent comments, add it to the parser's
   186  // comments list, and return it together with the line at which
   187  // the last comment in the group ends. A non-comment token or n
   188  // empty lines terminate a comment group.
   189  func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
   190  	var list []*ast.Comment
   191  	endline = p.file.Line(p.pos)
   192  	for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
   193  		var comment *ast.Comment
   194  		comment, endline = p.consumeComment()
   195  		list = append(list, comment)
   196  	}
   197  
   198  	// add comment group to the comments list
   199  	comments = &ast.CommentGroup{List: list}
   200  	p.comments = append(p.comments, comments)
   201  
   202  	return
   203  }
   204  
   205  // Advance to the next non-comment token. In the process, collect
   206  // any comment groups encountered, and remember the last lead and
   207  // line comments.
   208  //
   209  // A lead comment is a comment group that starts and ends in a
   210  // line without any other tokens and that is followed by a non-comment
   211  // token on the line immediately after the comment group.
   212  //
   213  // A line comment is a comment group that follows a non-comment
   214  // token on the same line, and that has no tokens after it on the line
   215  // where it ends.
   216  //
   217  // Lead and line comments may be considered documentation that is
   218  // stored in the AST.
   219  func (p *parser) next() {
   220  	p.leadComment = nil
   221  	p.lineComment = nil
   222  	prev := p.pos
   223  	p.next0()
   224  
   225  	if p.tok == token.COMMENT {
   226  		var comment *ast.CommentGroup
   227  		var endline int
   228  
   229  		if p.file.Line(p.pos) == p.file.Line(prev) {
   230  			// The comment is on same line as the previous token; it
   231  			// cannot be a lead comment but may be a line comment.
   232  			comment, endline = p.consumeCommentGroup(0)
   233  			if p.file.Line(p.pos) != endline || p.tok == token.SEMICOLON || p.tok == token.EOF {
   234  				// The next token is on a different line, thus
   235  				// the last comment group is a line comment.
   236  				p.lineComment = comment
   237  			}
   238  		}
   239  
   240  		// consume successor comments, if any
   241  		endline = -1
   242  		for p.tok == token.COMMENT {
   243  			comment, endline = p.consumeCommentGroup(1)
   244  		}
   245  
   246  		if endline+1 == p.file.Line(p.pos) {
   247  			// The next token is following on the line immediately after the
   248  			// comment group, thus the last comment group is a lead comment.
   249  			p.leadComment = comment
   250  		}
   251  	}
   252  }
   253  
   254  // A bailout panic is raised to indicate early termination. pos and msg are
   255  // only populated when bailing out of object resolution.
   256  type bailout struct {
   257  	pos token.Pos
   258  	msg string
   259  }
   260  
   261  func (p *parser) error(pos token.Pos, msg string) {
   262  	if p.trace {
   263  		defer un(trace(p, "error: "+msg))
   264  	}
   265  
   266  	epos := p.file.Position(pos)
   267  
   268  	// If AllErrors is not set, discard errors reported on the same line
   269  	// as the last recorded error and stop parsing if there are more than
   270  	// 10 errors.
   271  	if p.mode&AllErrors == 0 {
   272  		n := len(p.errors)
   273  		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
   274  			return // discard - likely a spurious error
   275  		}
   276  		if n > 10 {
   277  			panic(bailout{})
   278  		}
   279  	}
   280  
   281  	p.errors.Add(epos, msg)
   282  }
   283  
   284  func (p *parser) errorExpected(pos token.Pos, msg string) {
   285  	msg = "expected " + msg
   286  	if pos == p.pos {
   287  		// the error happened at the current position;
   288  		// make the error message more specific
   289  		switch {
   290  		case p.tok == token.SEMICOLON && p.lit == "\n":
   291  			msg += ", found newline"
   292  		case p.tok.IsLiteral():
   293  			// print 123 rather than 'INT', etc.
   294  			msg += ", found " + p.lit
   295  		default:
   296  			msg += ", found '" + p.tok.String() + "'"
   297  		}
   298  	}
   299  	p.error(pos, msg)
   300  }
   301  
   302  func (p *parser) expect(tok token.Token) token.Pos {
   303  	pos := p.pos
   304  	if p.tok != tok {
   305  		p.errorExpected(pos, "'"+tok.String()+"'")
   306  	}
   307  	p.next() // make progress
   308  	return pos
   309  }
   310  
   311  // expect2 is like expect, but it returns an invalid position
   312  // if the expected token is not found.
   313  func (p *parser) expect2(tok token.Token) (pos token.Pos) {
   314  	if p.tok == tok {
   315  		pos = p.pos
   316  	} else {
   317  		p.errorExpected(p.pos, "'"+tok.String()+"'")
   318  	}
   319  	p.next() // make progress
   320  	return
   321  }
   322  
   323  // expectClosing is like expect but provides a better error message
   324  // for the common case of a missing comma before a newline.
   325  func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
   326  	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
   327  		p.error(p.pos, "missing ',' before newline in "+context)
   328  		p.next()
   329  	}
   330  	return p.expect(tok)
   331  }
   332  
   333  // expectSemi consumes a semicolon and returns the applicable line comment.
   334  func (p *parser) expectSemi() (comment *ast.CommentGroup) {
   335  	// semicolon is optional before a closing ')' or '}'
   336  	if p.tok != token.RPAREN && p.tok != token.RBRACE {
   337  		switch p.tok {
   338  		case token.COMMA:
   339  			// permit a ',' instead of a ';' but complain
   340  			p.errorExpected(p.pos, "';'")
   341  			fallthrough
   342  		case token.SEMICOLON:
   343  			if p.lit == ";" {
   344  				// explicit semicolon
   345  				p.next()
   346  				comment = p.lineComment // use following comments
   347  			} else {
   348  				// artificial semicolon
   349  				comment = p.lineComment // use preceding comments
   350  				p.next()
   351  			}
   352  			return comment
   353  		default:
   354  			p.errorExpected(p.pos, "';'")
   355  			p.advance(stmtStart)
   356  		}
   357  	}
   358  	return nil
   359  }
   360  
   361  func (p *parser) atComma(context string, follow token.Token) bool {
   362  	if p.tok == token.COMMA {
   363  		return true
   364  	}
   365  	if p.tok != follow {
   366  		msg := "missing ','"
   367  		if p.tok == token.SEMICOLON && p.lit == "\n" {
   368  			msg += " before newline"
   369  		}
   370  		p.error(p.pos, msg+" in "+context)
   371  		return true // "insert" comma and continue
   372  	}
   373  	return false
   374  }
   375  
   376  func assert(cond bool, msg string) {
   377  	if !cond {
   378  		panic("go/parser internal error: " + msg)
   379  	}
   380  }
   381  
   382  // advance consumes tokens until the current token p.tok
   383  // is in the 'to' set, or token.EOF. For error recovery.
   384  func (p *parser) advance(to map[token.Token]bool) {
   385  	for ; p.tok != token.EOF; p.next() {
   386  		if to[p.tok] {
   387  			// Return only if parser made some progress since last
   388  			// sync or if it has not reached 10 advance calls without
   389  			// progress. Otherwise consume at least one token to
   390  			// avoid an endless parser loop (it is possible that
   391  			// both parseOperand and parseStmt call advance and
   392  			// correctly do not advance, thus the need for the
   393  			// invocation limit p.syncCnt).
   394  			if p.pos == p.syncPos && p.syncCnt < 10 {
   395  				p.syncCnt++
   396  				return
   397  			}
   398  			if p.pos > p.syncPos {
   399  				p.syncPos = p.pos
   400  				p.syncCnt = 0
   401  				return
   402  			}
   403  			// Reaching here indicates a parser bug, likely an
   404  			// incorrect token list in this function, but it only
   405  			// leads to skipping of possibly correct code if a
   406  			// previous error is present, and thus is preferred
   407  			// over a non-terminating parse.
   408  		}
   409  	}
   410  }
   411  
   412  var stmtStart = map[token.Token]bool{
   413  	token.BREAK:       true,
   414  	token.CONST:       true,
   415  	token.CONTINUE:    true,
   416  	token.DEFER:       true,
   417  	token.FALLTHROUGH: true,
   418  	token.FOR:         true,
   419  	token.GO:          true,
   420  	token.GOTO:        true,
   421  	token.IF:          true,
   422  	token.RETURN:      true,
   423  	token.SELECT:      true,
   424  	token.SWITCH:      true,
   425  	token.TYPE:        true,
   426  	token.VAR:         true,
   427  }
   428  
   429  var declStart = map[token.Token]bool{
   430  	token.IMPORT: true,
   431  	token.CONST:  true,
   432  	token.TYPE:   true,
   433  	token.VAR:    true,
   434  }
   435  
   436  var exprEnd = map[token.Token]bool{
   437  	token.COMMA:     true,
   438  	token.COLON:     true,
   439  	token.SEMICOLON: true,
   440  	token.RPAREN:    true,
   441  	token.RBRACK:    true,
   442  	token.RBRACE:    true,
   443  }
   444  
   445  // safePos returns a valid file position for a given position: If pos
   446  // is valid to begin with, safePos returns pos. If pos is out-of-range,
   447  // safePos returns the EOF position.
   448  //
   449  // This is hack to work around "artificial" end positions in the AST which
   450  // are computed by adding 1 to (presumably valid) token positions. If the
   451  // token positions are invalid due to parse errors, the resulting end position
   452  // may be past the file's EOF position, which would lead to panics if used
   453  // later on.
   454  func (p *parser) safePos(pos token.Pos) (res token.Pos) {
   455  	defer func() {
   456  		if recover() != nil {
   457  			res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
   458  		}
   459  	}()
   460  	_ = p.file.Offset(pos) // trigger a panic if position is out-of-range
   461  	return pos
   462  }
   463  
   464  // ----------------------------------------------------------------------------
   465  // Identifiers
   466  
   467  func (p *parser) parseIdent() *ast.Ident {
   468  	pos := p.pos
   469  	name := "_"
   470  	if p.tok == token.IDENT {
   471  		name = p.lit
   472  		p.next()
   473  	} else {
   474  		p.expect(token.IDENT) // use expect() error handling
   475  	}
   476  	return &ast.Ident{NamePos: pos, Name: name}
   477  }
   478  
   479  func (p *parser) parseIdentList() (list []*ast.Ident) {
   480  	if p.trace {
   481  		defer un(trace(p, "IdentList"))
   482  	}
   483  
   484  	list = append(list, p.parseIdent())
   485  	for p.tok == token.COMMA {
   486  		p.next()
   487  		list = append(list, p.parseIdent())
   488  	}
   489  
   490  	return
   491  }
   492  
   493  // ----------------------------------------------------------------------------
   494  // Common productions
   495  
   496  // If lhs is set, result list elements which are identifiers are not resolved.
   497  func (p *parser) parseExprList() (list []ast.Expr) {
   498  	if p.trace {
   499  		defer un(trace(p, "ExpressionList"))
   500  	}
   501  
   502  	list = append(list, p.parseExpr())
   503  	for p.tok == token.COMMA {
   504  		p.next()
   505  		list = append(list, p.parseExpr())
   506  	}
   507  
   508  	return
   509  }
   510  
   511  func (p *parser) parseList(inRhs bool) []ast.Expr {
   512  	old := p.inRhs
   513  	p.inRhs = inRhs
   514  	list := p.parseExprList()
   515  	p.inRhs = old
   516  	return list
   517  }
   518  
   519  // ----------------------------------------------------------------------------
   520  // Types
   521  
   522  func (p *parser) parseType() ast.Expr {
   523  	if p.trace {
   524  		defer un(trace(p, "Type"))
   525  	}
   526  
   527  	typ := p.tryIdentOrType()
   528  
   529  	if typ == nil {
   530  		pos := p.pos
   531  		p.errorExpected(pos, "type")
   532  		p.advance(exprEnd)
   533  		return &ast.BadExpr{From: pos, To: p.pos}
   534  	}
   535  
   536  	return typ
   537  }
   538  
   539  func (p *parser) parseQualifiedIdent(ident *ast.Ident) ast.Expr {
   540  	if p.trace {
   541  		defer un(trace(p, "QualifiedIdent"))
   542  	}
   543  
   544  	typ := p.parseTypeName(ident)
   545  	if p.tok == token.LBRACK {
   546  		typ = p.parseTypeInstance(typ)
   547  	}
   548  
   549  	return typ
   550  }
   551  
   552  // If the result is an identifier, it is not resolved.
   553  func (p *parser) parseTypeName(ident *ast.Ident) ast.Expr {
   554  	if p.trace {
   555  		defer un(trace(p, "TypeName"))
   556  	}
   557  
   558  	if ident == nil {
   559  		ident = p.parseIdent()
   560  	}
   561  
   562  	if p.tok == token.PERIOD {
   563  		// ident is a package name
   564  		p.next()
   565  		sel := p.parseIdent()
   566  		return &ast.SelectorExpr{X: ident, Sel: sel}
   567  	}
   568  
   569  	return ident
   570  }
   571  
   572  // "[" has already been consumed, and lbrack is its position.
   573  // If len != nil it is the already consumed array length.
   574  func (p *parser) parseArrayType(lbrack token.Pos, len ast.Expr) *ast.ArrayType {
   575  	if p.trace {
   576  		defer un(trace(p, "ArrayType"))
   577  	}
   578  
   579  	if len == nil {
   580  		p.exprLev++
   581  		// always permit ellipsis for more fault-tolerant parsing
   582  		if p.tok == token.ELLIPSIS {
   583  			len = &ast.Ellipsis{Ellipsis: p.pos}
   584  			p.next()
   585  		} else if p.tok != token.RBRACK {
   586  			len = p.parseRhs()
   587  		}
   588  		p.exprLev--
   589  	}
   590  	if p.tok == token.COMMA {
   591  		// Trailing commas are accepted in type parameter
   592  		// lists but not in array type declarations.
   593  		// Accept for better error handling but complain.
   594  		p.error(p.pos, "unexpected comma; expecting ]")
   595  		p.next()
   596  	}
   597  	p.expect(token.RBRACK)
   598  	elt := p.parseType()
   599  	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
   600  }
   601  
   602  func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Expr) {
   603  	if p.trace {
   604  		defer un(trace(p, "ArrayFieldOrTypeInstance"))
   605  	}
   606  
   607  	lbrack := p.expect(token.LBRACK)
   608  	trailingComma := token.NoPos // if valid, the position of a trailing comma preceding the ']'
   609  	var args []ast.Expr
   610  	if p.tok != token.RBRACK {
   611  		p.exprLev++
   612  		args = append(args, p.parseRhs())
   613  		for p.tok == token.COMMA {
   614  			comma := p.pos
   615  			p.next()
   616  			if p.tok == token.RBRACK {
   617  				trailingComma = comma
   618  				break
   619  			}
   620  			args = append(args, p.parseRhs())
   621  		}
   622  		p.exprLev--
   623  	}
   624  	rbrack := p.expect(token.RBRACK)
   625  
   626  	if len(args) == 0 {
   627  		// x []E
   628  		elt := p.parseType()
   629  		return x, &ast.ArrayType{Lbrack: lbrack, Elt: elt}
   630  	}
   631  
   632  	// x [P]E or x[P]
   633  	if len(args) == 1 {
   634  		elt := p.tryIdentOrType()
   635  		if elt != nil {
   636  			// x [P]E
   637  			if trailingComma.IsValid() {
   638  				// Trailing commas are invalid in array type fields.
   639  				p.error(trailingComma, "unexpected comma; expecting ]")
   640  			}
   641  			return x, &ast.ArrayType{Lbrack: lbrack, Len: args[0], Elt: elt}
   642  		}
   643  	}
   644  
   645  	// x[P], x[P1, P2], ...
   646  	return nil, typeparams.PackIndexExpr(x, lbrack, args, rbrack)
   647  }
   648  
   649  func (p *parser) parseFieldDecl() *ast.Field {
   650  	if p.trace {
   651  		defer un(trace(p, "FieldDecl"))
   652  	}
   653  
   654  	doc := p.leadComment
   655  
   656  	var names []*ast.Ident
   657  	var typ ast.Expr
   658  	switch p.tok {
   659  	case token.IDENT:
   660  		name := p.parseIdent()
   661  		if p.tok == token.PERIOD || p.tok == token.STRING || p.tok == token.SEMICOLON || p.tok == token.RBRACE {
   662  			// embedded type
   663  			typ = name
   664  			if p.tok == token.PERIOD {
   665  				typ = p.parseQualifiedIdent(name)
   666  			}
   667  		} else {
   668  			// name1, name2, ... T
   669  			names = []*ast.Ident{name}
   670  			for p.tok == token.COMMA {
   671  				p.next()
   672  				names = append(names, p.parseIdent())
   673  			}
   674  			// Careful dance: We don't know if we have an embedded instantiated
   675  			// type T[P1, P2, ...] or a field T of array type []E or [P]E.
   676  			if len(names) == 1 && p.tok == token.LBRACK {
   677  				name, typ = p.parseArrayFieldOrTypeInstance(name)
   678  				if name == nil {
   679  					names = nil
   680  				}
   681  			} else {
   682  				// T P
   683  				typ = p.parseType()
   684  			}
   685  		}
   686  	case token.MUL:
   687  		star := p.pos
   688  		p.next()
   689  		if p.tok == token.LPAREN {
   690  			// *(T)
   691  			p.error(p.pos, "cannot parenthesize embedded type")
   692  			p.next()
   693  			typ = p.parseQualifiedIdent(nil)
   694  			// expect closing ')' but no need to complain if missing
   695  			if p.tok == token.RPAREN {
   696  				p.next()
   697  			}
   698  		} else {
   699  			// *T
   700  			typ = p.parseQualifiedIdent(nil)
   701  		}
   702  		typ = &ast.StarExpr{Star: star, X: typ}
   703  
   704  	case token.LPAREN:
   705  		p.error(p.pos, "cannot parenthesize embedded type")
   706  		p.next()
   707  		if p.tok == token.MUL {
   708  			// (*T)
   709  			star := p.pos
   710  			p.next()
   711  			typ = &ast.StarExpr{Star: star, X: p.parseQualifiedIdent(nil)}
   712  		} else {
   713  			// (T)
   714  			typ = p.parseQualifiedIdent(nil)
   715  		}
   716  		// expect closing ')' but no need to complain if missing
   717  		if p.tok == token.RPAREN {
   718  			p.next()
   719  		}
   720  
   721  	default:
   722  		pos := p.pos
   723  		p.errorExpected(pos, "field name or embedded type")
   724  		p.advance(exprEnd)
   725  		typ = &ast.BadExpr{From: pos, To: p.pos}
   726  	}
   727  
   728  	var tag *ast.BasicLit
   729  	if p.tok == token.STRING {
   730  		tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
   731  		p.next()
   732  	}
   733  
   734  	comment := p.expectSemi()
   735  
   736  	field := &ast.Field{Doc: doc, Names: names, Type: typ, Tag: tag, Comment: comment}
   737  	return field
   738  }
   739  
   740  func (p *parser) parseStructType() *ast.StructType {
   741  	if p.trace {
   742  		defer un(trace(p, "StructType"))
   743  	}
   744  
   745  	pos := p.expect(token.STRUCT)
   746  	lbrace := p.expect(token.LBRACE)
   747  	var list []*ast.Field
   748  	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
   749  		// a field declaration cannot start with a '(' but we accept
   750  		// it here for more robust parsing and better error messages
   751  		// (parseFieldDecl will check and complain if necessary)
   752  		list = append(list, p.parseFieldDecl())
   753  	}
   754  	rbrace := p.expect(token.RBRACE)
   755  
   756  	return &ast.StructType{
   757  		Struct: pos,
   758  		Fields: &ast.FieldList{
   759  			Opening: lbrace,
   760  			List:    list,
   761  			Closing: rbrace,
   762  		},
   763  	}
   764  }
   765  
   766  func (p *parser) parsePointerType() *ast.StarExpr {
   767  	if p.trace {
   768  		defer un(trace(p, "PointerType"))
   769  	}
   770  
   771  	star := p.expect(token.MUL)
   772  	base := p.parseType()
   773  
   774  	return &ast.StarExpr{Star: star, X: base}
   775  }
   776  
   777  func (p *parser) parseDotsType() *ast.Ellipsis {
   778  	if p.trace {
   779  		defer un(trace(p, "DotsType"))
   780  	}
   781  
   782  	pos := p.expect(token.ELLIPSIS)
   783  	elt := p.parseType()
   784  
   785  	return &ast.Ellipsis{Ellipsis: pos, Elt: elt}
   786  }
   787  
   788  type field struct {
   789  	name *ast.Ident
   790  	typ  ast.Expr
   791  }
   792  
   793  func (p *parser) parseParamDecl(name *ast.Ident, typeSetsOK bool) (f field) {
   794  	// TODO(rFindley) refactor to be more similar to paramDeclOrNil in the syntax
   795  	// package
   796  	if p.trace {
   797  		defer un(trace(p, "ParamDeclOrNil"))
   798  	}
   799  
   800  	ptok := p.tok
   801  	if name != nil {
   802  		p.tok = token.IDENT // force token.IDENT case in switch below
   803  	} else if typeSetsOK && p.tok == token.TILDE {
   804  		// "~" ...
   805  		return field{nil, p.embeddedElem(nil)}
   806  	}
   807  
   808  	switch p.tok {
   809  	case token.IDENT:
   810  		// name
   811  		if name != nil {
   812  			f.name = name
   813  			p.tok = ptok
   814  		} else {
   815  			f.name = p.parseIdent()
   816  		}
   817  		switch p.tok {
   818  		case token.IDENT, token.MUL, token.ARROW, token.FUNC, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
   819  			// name type
   820  			f.typ = p.parseType()
   821  
   822  		case token.LBRACK:
   823  			// name "[" type1, ..., typeN "]" or name "[" n "]" type
   824  			f.name, f.typ = p.parseArrayFieldOrTypeInstance(f.name)
   825  
   826  		case token.ELLIPSIS:
   827  			// name "..." type
   828  			f.typ = p.parseDotsType()
   829  			return // don't allow ...type "|" ...
   830  
   831  		case token.PERIOD:
   832  			// name "." ...
   833  			f.typ = p.parseQualifiedIdent(f.name)
   834  			f.name = nil
   835  
   836  		case token.TILDE:
   837  			if typeSetsOK {
   838  				f.typ = p.embeddedElem(nil)
   839  				return
   840  			}
   841  
   842  		case token.OR:
   843  			if typeSetsOK {
   844  				// name "|" typeset
   845  				f.typ = p.embeddedElem(f.name)
   846  				f.name = nil
   847  				return
   848  			}
   849  		}
   850  
   851  	case token.MUL, token.ARROW, token.FUNC, token.LBRACK, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
   852  		// type
   853  		f.typ = p.parseType()
   854  
   855  	case token.ELLIPSIS:
   856  		// "..." type
   857  		// (always accepted)
   858  		f.typ = p.parseDotsType()
   859  		return // don't allow ...type "|" ...
   860  
   861  	default:
   862  		// TODO(rfindley): this is incorrect in the case of type parameter lists
   863  		//                 (should be "']'" in that case)
   864  		p.errorExpected(p.pos, "')'")
   865  		p.advance(exprEnd)
   866  	}
   867  
   868  	// [name] type "|"
   869  	if typeSetsOK && p.tok == token.OR && f.typ != nil {
   870  		f.typ = p.embeddedElem(f.typ)
   871  	}
   872  
   873  	return
   874  }
   875  
   876  func (p *parser) parseParameterList(name0 *ast.Ident, typ0 ast.Expr, closing token.Token) (params []*ast.Field) {
   877  	if p.trace {
   878  		defer un(trace(p, "ParameterList"))
   879  	}
   880  
   881  	// Type parameters are the only parameter list closed by ']'.
   882  	tparams := closing == token.RBRACK
   883  
   884  	pos0 := p.pos
   885  	if name0 != nil {
   886  		pos0 = name0.Pos()
   887  	} else if typ0 != nil {
   888  		pos0 = typ0.Pos()
   889  	}
   890  
   891  	// Note: The code below matches the corresponding code in the syntax
   892  	//       parser closely. Changes must be reflected in either parser.
   893  	//       For the code to match, we use the local []field list that
   894  	//       corresponds to []syntax.Field. At the end, the list must be
   895  	//       converted into an []*ast.Field.
   896  
   897  	var list []field
   898  	var named int // number of parameters that have an explicit name and type
   899  	var typed int // number of parameters that have an explicit type
   900  
   901  	for name0 != nil || p.tok != closing && p.tok != token.EOF {
   902  		var par field
   903  		if typ0 != nil {
   904  			if tparams {
   905  				typ0 = p.embeddedElem(typ0)
   906  			}
   907  			par = field{name0, typ0}
   908  		} else {
   909  			par = p.parseParamDecl(name0, tparams)
   910  		}
   911  		name0 = nil // 1st name was consumed if present
   912  		typ0 = nil  // 1st typ was consumed if present
   913  		if par.name != nil || par.typ != nil {
   914  			list = append(list, par)
   915  			if par.name != nil && par.typ != nil {
   916  				named++
   917  			}
   918  			if par.typ != nil {
   919  				typed++
   920  			}
   921  		}
   922  		if !p.atComma("parameter list", closing) {
   923  			break
   924  		}
   925  		p.next()
   926  	}
   927  
   928  	if len(list) == 0 {
   929  		return // not uncommon
   930  	}
   931  
   932  	// distribute parameter types (len(list) > 0)
   933  	if named == 0 {
   934  		// all unnamed => found names are type names
   935  		for i := 0; i < len(list); i++ {
   936  			par := &list[i]
   937  			if typ := par.name; typ != nil {
   938  				par.typ = typ
   939  				par.name = nil
   940  			}
   941  		}
   942  		if tparams {
   943  			// This is the same error handling as below, adjusted for type parameters only.
   944  			// See comment below for details. (go.dev/issue/64534)
   945  			var errPos token.Pos
   946  			var msg string
   947  			if named == typed /* same as typed == 0 */ {
   948  				errPos = p.pos // position error at closing ]
   949  				msg = "missing type constraint"
   950  			} else {
   951  				errPos = pos0 // position at opening [ or first name
   952  				msg = "missing type parameter name"
   953  				if len(list) == 1 {
   954  					msg += " or invalid array length"
   955  				}
   956  			}
   957  			p.error(errPos, msg)
   958  		}
   959  	} else if named != len(list) {
   960  		// some named or we're in a type parameter list => all must be named
   961  		var errPos token.Pos // left-most error position (or invalid)
   962  		var typ ast.Expr     // current type (from right to left)
   963  		for i := len(list) - 1; i >= 0; i-- {
   964  			if par := &list[i]; par.typ != nil {
   965  				typ = par.typ
   966  				if par.name == nil {
   967  					errPos = typ.Pos()
   968  					n := ast.NewIdent("_")
   969  					n.NamePos = errPos // correct position
   970  					par.name = n
   971  				}
   972  			} else if typ != nil {
   973  				par.typ = typ
   974  			} else {
   975  				// par.typ == nil && typ == nil => we only have a par.name
   976  				errPos = par.name.Pos()
   977  				par.typ = &ast.BadExpr{From: errPos, To: p.pos}
   978  			}
   979  		}
   980  		if errPos.IsValid() {
   981  			var msg string
   982  			if tparams {
   983  				// Not all parameters are named because named != len(list).
   984  				// If named == typed we must have parameters that have no types,
   985  				// and they must be at the end of the parameter list, otherwise
   986  				// the types would have been filled in by the right-to-left sweep
   987  				// above and we wouldn't have an error. Since we are in a type
   988  				// parameter list, the missing types are constraints.
   989  				if named == typed {
   990  					errPos = p.pos // position error at closing ]
   991  					msg = "missing type constraint"
   992  				} else {
   993  					msg = "missing type parameter name"
   994  					// go.dev/issue/60812
   995  					if len(list) == 1 {
   996  						msg += " or invalid array length"
   997  					}
   998  				}
   999  			} else {
  1000  				msg = "mixed named and unnamed parameters"
  1001  			}
  1002  			p.error(errPos, msg)
  1003  		}
  1004  	}
  1005  
  1006  	// Convert list to []*ast.Field.
  1007  	// If list contains types only, each type gets its own ast.Field.
  1008  	if named == 0 {
  1009  		// parameter list consists of types only
  1010  		for _, par := range list {
  1011  			assert(par.typ != nil, "nil type in unnamed parameter list")
  1012  			params = append(params, &ast.Field{Type: par.typ})
  1013  		}
  1014  		return
  1015  	}
  1016  
  1017  	// If the parameter list consists of named parameters with types,
  1018  	// collect all names with the same types into a single ast.Field.
  1019  	var names []*ast.Ident
  1020  	var typ ast.Expr
  1021  	addParams := func() {
  1022  		assert(typ != nil, "nil type in named parameter list")
  1023  		field := &ast.Field{Names: names, Type: typ}
  1024  		params = append(params, field)
  1025  		names = nil
  1026  	}
  1027  	for _, par := range list {
  1028  		if par.typ != typ {
  1029  			if len(names) > 0 {
  1030  				addParams()
  1031  			}
  1032  			typ = par.typ
  1033  		}
  1034  		names = append(names, par.name)
  1035  	}
  1036  	if len(names) > 0 {
  1037  		addParams()
  1038  	}
  1039  	return
  1040  }
  1041  
  1042  func (p *parser) parseParameters(acceptTParams bool) (tparams, params *ast.FieldList) {
  1043  	if p.trace {
  1044  		defer un(trace(p, "Parameters"))
  1045  	}
  1046  
  1047  	if acceptTParams && p.tok == token.LBRACK {
  1048  		opening := p.pos
  1049  		p.next()
  1050  		// [T any](params) syntax
  1051  		list := p.parseParameterList(nil, nil, token.RBRACK)
  1052  		rbrack := p.expect(token.RBRACK)
  1053  		tparams = &ast.FieldList{Opening: opening, List: list, Closing: rbrack}
  1054  		// Type parameter lists must not be empty.
  1055  		if tparams.NumFields() == 0 {
  1056  			p.error(tparams.Closing, "empty type parameter list")
  1057  			tparams = nil // avoid follow-on errors
  1058  		}
  1059  	}
  1060  
  1061  	opening := p.expect(token.LPAREN)
  1062  
  1063  	var fields []*ast.Field
  1064  	if p.tok != token.RPAREN {
  1065  		fields = p.parseParameterList(nil, nil, token.RPAREN)
  1066  	}
  1067  
  1068  	rparen := p.expect(token.RPAREN)
  1069  	params = &ast.FieldList{Opening: opening, List: fields, Closing: rparen}
  1070  
  1071  	return
  1072  }
  1073  
  1074  func (p *parser) parseResult() *ast.FieldList {
  1075  	if p.trace {
  1076  		defer un(trace(p, "Result"))
  1077  	}
  1078  
  1079  	if p.tok == token.LPAREN {
  1080  		_, results := p.parseParameters(false)
  1081  		return results
  1082  	}
  1083  
  1084  	typ := p.tryIdentOrType()
  1085  	if typ != nil {
  1086  		list := make([]*ast.Field, 1)
  1087  		list[0] = &ast.Field{Type: typ}
  1088  		return &ast.FieldList{List: list}
  1089  	}
  1090  
  1091  	return nil
  1092  }
  1093  
  1094  func (p *parser) parseFuncType() *ast.FuncType {
  1095  	if p.trace {
  1096  		defer un(trace(p, "FuncType"))
  1097  	}
  1098  
  1099  	pos := p.expect(token.FUNC)
  1100  	tparams, params := p.parseParameters(true)
  1101  	if tparams != nil {
  1102  		p.error(tparams.Pos(), "function type must have no type parameters")
  1103  	}
  1104  	results := p.parseResult()
  1105  
  1106  	return &ast.FuncType{Func: pos, Params: params, Results: results}
  1107  }
  1108  
  1109  func (p *parser) parseMethodSpec() *ast.Field {
  1110  	if p.trace {
  1111  		defer un(trace(p, "MethodSpec"))
  1112  	}
  1113  
  1114  	doc := p.leadComment
  1115  	var idents []*ast.Ident
  1116  	var typ ast.Expr
  1117  	x := p.parseTypeName(nil)
  1118  	if ident, _ := x.(*ast.Ident); ident != nil {
  1119  		switch {
  1120  		case p.tok == token.LBRACK:
  1121  			// generic method or embedded instantiated type
  1122  			lbrack := p.pos
  1123  			p.next()
  1124  			p.exprLev++
  1125  			x := p.parseExpr()
  1126  			p.exprLev--
  1127  			if name0, _ := x.(*ast.Ident); name0 != nil && p.tok != token.COMMA && p.tok != token.RBRACK {
  1128  				// generic method m[T any]
  1129  				//
  1130  				// Interface methods do not have type parameters. We parse them for a
  1131  				// better error message and improved error recovery.
  1132  				_ = p.parseParameterList(name0, nil, token.RBRACK)
  1133  				_ = p.expect(token.RBRACK)
  1134  				p.error(lbrack, "interface method must have no type parameters")
  1135  
  1136  				// TODO(rfindley) refactor to share code with parseFuncType.
  1137  				_, params := p.parseParameters(false)
  1138  				results := p.parseResult()
  1139  				idents = []*ast.Ident{ident}
  1140  				typ = &ast.FuncType{
  1141  					Func:    token.NoPos,
  1142  					Params:  params,
  1143  					Results: results,
  1144  				}
  1145  			} else {
  1146  				// embedded instantiated type
  1147  				// TODO(rfindley) should resolve all identifiers in x.
  1148  				list := []ast.Expr{x}
  1149  				if p.atComma("type argument list", token.RBRACK) {
  1150  					p.exprLev++
  1151  					p.next()
  1152  					for p.tok != token.RBRACK && p.tok != token.EOF {
  1153  						list = append(list, p.parseType())
  1154  						if !p.atComma("type argument list", token.RBRACK) {
  1155  							break
  1156  						}
  1157  						p.next()
  1158  					}
  1159  					p.exprLev--
  1160  				}
  1161  				rbrack := p.expectClosing(token.RBRACK, "type argument list")
  1162  				typ = typeparams.PackIndexExpr(ident, lbrack, list, rbrack)
  1163  			}
  1164  		case p.tok == token.LPAREN:
  1165  			// ordinary method
  1166  			// TODO(rfindley) refactor to share code with parseFuncType.
  1167  			_, params := p.parseParameters(false)
  1168  			results := p.parseResult()
  1169  			idents = []*ast.Ident{ident}
  1170  			typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
  1171  		default:
  1172  			// embedded type
  1173  			typ = x
  1174  		}
  1175  	} else {
  1176  		// embedded, possibly instantiated type
  1177  		typ = x
  1178  		if p.tok == token.LBRACK {
  1179  			// embedded instantiated interface
  1180  			typ = p.parseTypeInstance(typ)
  1181  		}
  1182  	}
  1183  
  1184  	// Comment is added at the callsite: the field below may joined with
  1185  	// additional type specs using '|'.
  1186  	// TODO(rfindley) this should be refactored.
  1187  	// TODO(rfindley) add more tests for comment handling.
  1188  	return &ast.Field{Doc: doc, Names: idents, Type: typ}
  1189  }
  1190  
  1191  func (p *parser) embeddedElem(x ast.Expr) ast.Expr {
  1192  	if p.trace {
  1193  		defer un(trace(p, "EmbeddedElem"))
  1194  	}
  1195  	if x == nil {
  1196  		x = p.embeddedTerm()
  1197  	}
  1198  	for p.tok == token.OR {
  1199  		t := new(ast.BinaryExpr)
  1200  		t.OpPos = p.pos
  1201  		t.Op = token.OR
  1202  		p.next()
  1203  		t.X = x
  1204  		t.Y = p.embeddedTerm()
  1205  		x = t
  1206  	}
  1207  	return x
  1208  }
  1209  
  1210  func (p *parser) embeddedTerm() ast.Expr {
  1211  	if p.trace {
  1212  		defer un(trace(p, "EmbeddedTerm"))
  1213  	}
  1214  	if p.tok == token.TILDE {
  1215  		t := new(ast.UnaryExpr)
  1216  		t.OpPos = p.pos
  1217  		t.Op = token.TILDE
  1218  		p.next()
  1219  		t.X = p.parseType()
  1220  		return t
  1221  	}
  1222  
  1223  	t := p.tryIdentOrType()
  1224  	if t == nil {
  1225  		pos := p.pos
  1226  		p.errorExpected(pos, "~ term or type")
  1227  		p.advance(exprEnd)
  1228  		return &ast.BadExpr{From: pos, To: p.pos}
  1229  	}
  1230  
  1231  	return t
  1232  }
  1233  
  1234  func (p *parser) parseInterfaceType() *ast.InterfaceType {
  1235  	if p.trace {
  1236  		defer un(trace(p, "InterfaceType"))
  1237  	}
  1238  
  1239  	pos := p.expect(token.INTERFACE)
  1240  	lbrace := p.expect(token.LBRACE)
  1241  
  1242  	var list []*ast.Field
  1243  
  1244  parseElements:
  1245  	for {
  1246  		switch {
  1247  		case p.tok == token.IDENT:
  1248  			f := p.parseMethodSpec()
  1249  			if f.Names == nil {
  1250  				f.Type = p.embeddedElem(f.Type)
  1251  			}
  1252  			f.Comment = p.expectSemi()
  1253  			list = append(list, f)
  1254  		case p.tok == token.TILDE:
  1255  			typ := p.embeddedElem(nil)
  1256  			comment := p.expectSemi()
  1257  			list = append(list, &ast.Field{Type: typ, Comment: comment})
  1258  		default:
  1259  			if t := p.tryIdentOrType(); t != nil {
  1260  				typ := p.embeddedElem(t)
  1261  				comment := p.expectSemi()
  1262  				list = append(list, &ast.Field{Type: typ, Comment: comment})
  1263  			} else {
  1264  				break parseElements
  1265  			}
  1266  		}
  1267  	}
  1268  
  1269  	// TODO(rfindley): the error produced here could be improved, since we could
  1270  	// accept an identifier, 'type', or a '}' at this point.
  1271  	rbrace := p.expect(token.RBRACE)
  1272  
  1273  	return &ast.InterfaceType{
  1274  		Interface: pos,
  1275  		Methods: &ast.FieldList{
  1276  			Opening: lbrace,
  1277  			List:    list,
  1278  			Closing: rbrace,
  1279  		},
  1280  	}
  1281  }
  1282  
  1283  func (p *parser) parseMapType() *ast.MapType {
  1284  	if p.trace {
  1285  		defer un(trace(p, "MapType"))
  1286  	}
  1287  
  1288  	pos := p.expect(token.MAP)
  1289  	p.expect(token.LBRACK)
  1290  	key := p.parseType()
  1291  	p.expect(token.RBRACK)
  1292  	value := p.parseType()
  1293  
  1294  	return &ast.MapType{Map: pos, Key: key, Value: value}
  1295  }
  1296  
  1297  func (p *parser) parseChanType() *ast.ChanType {
  1298  	if p.trace {
  1299  		defer un(trace(p, "ChanType"))
  1300  	}
  1301  
  1302  	pos := p.pos
  1303  	dir := ast.SEND | ast.RECV
  1304  	var arrow token.Pos
  1305  	if p.tok == token.CHAN {
  1306  		p.next()
  1307  		if p.tok == token.ARROW {
  1308  			arrow = p.pos
  1309  			p.next()
  1310  			dir = ast.SEND
  1311  		}
  1312  	} else {
  1313  		arrow = p.expect(token.ARROW)
  1314  		p.expect(token.CHAN)
  1315  		dir = ast.RECV
  1316  	}
  1317  	value := p.parseType()
  1318  
  1319  	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
  1320  }
  1321  
  1322  func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
  1323  	if p.trace {
  1324  		defer un(trace(p, "TypeInstance"))
  1325  	}
  1326  
  1327  	opening := p.expect(token.LBRACK)
  1328  	p.exprLev++
  1329  	var list []ast.Expr
  1330  	for p.tok != token.RBRACK && p.tok != token.EOF {
  1331  		list = append(list, p.parseType())
  1332  		if !p.atComma("type argument list", token.RBRACK) {
  1333  			break
  1334  		}
  1335  		p.next()
  1336  	}
  1337  	p.exprLev--
  1338  
  1339  	closing := p.expectClosing(token.RBRACK, "type argument list")
  1340  
  1341  	if len(list) == 0 {
  1342  		p.errorExpected(closing, "type argument list")
  1343  		return &ast.IndexExpr{
  1344  			X:      typ,
  1345  			Lbrack: opening,
  1346  			Index:  &ast.BadExpr{From: opening + 1, To: closing},
  1347  			Rbrack: closing,
  1348  		}
  1349  	}
  1350  
  1351  	return typeparams.PackIndexExpr(typ, opening, list, closing)
  1352  }
  1353  
  1354  func (p *parser) tryIdentOrType() ast.Expr {
  1355  	defer decNestLev(incNestLev(p))
  1356  
  1357  	switch p.tok {
  1358  	case token.IDENT:
  1359  		typ := p.parseTypeName(nil)
  1360  		if p.tok == token.LBRACK {
  1361  			typ = p.parseTypeInstance(typ)
  1362  		}
  1363  		return typ
  1364  	case token.LBRACK:
  1365  		lbrack := p.expect(token.LBRACK)
  1366  		return p.parseArrayType(lbrack, nil)
  1367  	case token.STRUCT:
  1368  		return p.parseStructType()
  1369  	case token.MUL:
  1370  		return p.parsePointerType()
  1371  	case token.FUNC:
  1372  		return p.parseFuncType()
  1373  	case token.INTERFACE:
  1374  		return p.parseInterfaceType()
  1375  	case token.MAP:
  1376  		return p.parseMapType()
  1377  	case token.CHAN, token.ARROW:
  1378  		return p.parseChanType()
  1379  	case token.LPAREN:
  1380  		lparen := p.pos
  1381  		p.next()
  1382  		typ := p.parseType()
  1383  		rparen := p.expect(token.RPAREN)
  1384  		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
  1385  	}
  1386  
  1387  	// no type found
  1388  	return nil
  1389  }
  1390  
  1391  // ----------------------------------------------------------------------------
  1392  // Blocks
  1393  
  1394  func (p *parser) parseStmtList() (list []ast.Stmt) {
  1395  	if p.trace {
  1396  		defer un(trace(p, "StatementList"))
  1397  	}
  1398  
  1399  	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
  1400  		list = append(list, p.parseStmt())
  1401  	}
  1402  
  1403  	return
  1404  }
  1405  
  1406  func (p *parser) parseBody() *ast.BlockStmt {
  1407  	if p.trace {
  1408  		defer un(trace(p, "Body"))
  1409  	}
  1410  
  1411  	lbrace := p.expect(token.LBRACE)
  1412  	list := p.parseStmtList()
  1413  	rbrace := p.expect2(token.RBRACE)
  1414  
  1415  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1416  }
  1417  
  1418  func (p *parser) parseBlockStmt() *ast.BlockStmt {
  1419  	if p.trace {
  1420  		defer un(trace(p, "BlockStmt"))
  1421  	}
  1422  
  1423  	lbrace := p.expect(token.LBRACE)
  1424  	list := p.parseStmtList()
  1425  	rbrace := p.expect2(token.RBRACE)
  1426  
  1427  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1428  }
  1429  
  1430  // ----------------------------------------------------------------------------
  1431  // Expressions
  1432  
  1433  func (p *parser) parseFuncTypeOrLit() ast.Expr {
  1434  	if p.trace {
  1435  		defer un(trace(p, "FuncTypeOrLit"))
  1436  	}
  1437  
  1438  	typ := p.parseFuncType()
  1439  	if p.tok != token.LBRACE {
  1440  		// function type only
  1441  		return typ
  1442  	}
  1443  
  1444  	p.exprLev++
  1445  	body := p.parseBody()
  1446  	p.exprLev--
  1447  
  1448  	return &ast.FuncLit{Type: typ, Body: body}
  1449  }
  1450  
  1451  // parseOperand may return an expression or a raw type (incl. array
  1452  // types of the form [...]T). Callers must verify the result.
  1453  func (p *parser) parseOperand() ast.Expr {
  1454  	if p.trace {
  1455  		defer un(trace(p, "Operand"))
  1456  	}
  1457  
  1458  	switch p.tok {
  1459  	case token.IDENT:
  1460  		x := p.parseIdent()
  1461  		return x
  1462  
  1463  	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
  1464  		x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
  1465  		p.next()
  1466  		return x
  1467  
  1468  	case token.LPAREN:
  1469  		lparen := p.pos
  1470  		p.next()
  1471  		p.exprLev++
  1472  		x := p.parseRhs() // types may be parenthesized: (some type)
  1473  		p.exprLev--
  1474  		rparen := p.expect(token.RPAREN)
  1475  		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}
  1476  
  1477  	case token.FUNC:
  1478  		return p.parseFuncTypeOrLit()
  1479  	}
  1480  
  1481  	if typ := p.tryIdentOrType(); typ != nil { // do not consume trailing type parameters
  1482  		// could be type for composite literal or conversion
  1483  		_, isIdent := typ.(*ast.Ident)
  1484  		assert(!isIdent, "type cannot be identifier")
  1485  		return typ
  1486  	}
  1487  
  1488  	// we have an error
  1489  	pos := p.pos
  1490  	p.errorExpected(pos, "operand")
  1491  	p.advance(stmtStart)
  1492  	return &ast.BadExpr{From: pos, To: p.pos}
  1493  }
  1494  
  1495  func (p *parser) parseSelector(x ast.Expr) ast.Expr {
  1496  	if p.trace {
  1497  		defer un(trace(p, "Selector"))
  1498  	}
  1499  
  1500  	sel := p.parseIdent()
  1501  
  1502  	return &ast.SelectorExpr{X: x, Sel: sel}
  1503  }
  1504  
  1505  func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
  1506  	if p.trace {
  1507  		defer un(trace(p, "TypeAssertion"))
  1508  	}
  1509  
  1510  	lparen := p.expect(token.LPAREN)
  1511  	var typ ast.Expr
  1512  	if p.tok == token.TYPE {
  1513  		// type switch: typ == nil
  1514  		p.next()
  1515  	} else {
  1516  		typ = p.parseType()
  1517  	}
  1518  	rparen := p.expect(token.RPAREN)
  1519  
  1520  	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
  1521  }
  1522  
  1523  func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
  1524  	if p.trace {
  1525  		defer un(trace(p, "parseIndexOrSliceOrInstance"))
  1526  	}
  1527  
  1528  	lbrack := p.expect(token.LBRACK)
  1529  	if p.tok == token.RBRACK {
  1530  		// empty index, slice or index expressions are not permitted;
  1531  		// accept them for parsing tolerance, but complain
  1532  		p.errorExpected(p.pos, "operand")
  1533  		rbrack := p.pos
  1534  		p.next()
  1535  		return &ast.IndexExpr{
  1536  			X:      x,
  1537  			Lbrack: lbrack,
  1538  			Index:  &ast.BadExpr{From: rbrack, To: rbrack},
  1539  			Rbrack: rbrack,
  1540  		}
  1541  	}
  1542  	p.exprLev++
  1543  
  1544  	const N = 3 // change the 3 to 2 to disable 3-index slices
  1545  	var args []ast.Expr
  1546  	var index [N]ast.Expr
  1547  	var colons [N - 1]token.Pos
  1548  	if p.tok != token.COLON {
  1549  		// We can't know if we have an index expression or a type instantiation;
  1550  		// so even if we see a (named) type we are not going to be in type context.
  1551  		index[0] = p.parseRhs()
  1552  	}
  1553  	ncolons := 0
  1554  	switch p.tok {
  1555  	case token.COLON:
  1556  		// slice expression
  1557  		for p.tok == token.COLON && ncolons < len(colons) {
  1558  			colons[ncolons] = p.pos
  1559  			ncolons++
  1560  			p.next()
  1561  			if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
  1562  				index[ncolons] = p.parseRhs()
  1563  			}
  1564  		}
  1565  	case token.COMMA:
  1566  		// instance expression
  1567  		args = append(args, index[0])
  1568  		for p.tok == token.COMMA {
  1569  			p.next()
  1570  			if p.tok != token.RBRACK && p.tok != token.EOF {
  1571  				args = append(args, p.parseType())
  1572  			}
  1573  		}
  1574  	}
  1575  
  1576  	p.exprLev--
  1577  	rbrack := p.expect(token.RBRACK)
  1578  
  1579  	if ncolons > 0 {
  1580  		// slice expression
  1581  		slice3 := false
  1582  		if ncolons == 2 {
  1583  			slice3 = true
  1584  			// Check presence of middle and final index here rather than during type-checking
  1585  			// to prevent erroneous programs from passing through gofmt (was go.dev/issue/7305).
  1586  			if index[1] == nil {
  1587  				p.error(colons[0], "middle index required in 3-index slice")
  1588  				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
  1589  			}
  1590  			if index[2] == nil {
  1591  				p.error(colons[1], "final index required in 3-index slice")
  1592  				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
  1593  			}
  1594  		}
  1595  		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
  1596  	}
  1597  
  1598  	if len(args) == 0 {
  1599  		// index expression
  1600  		return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
  1601  	}
  1602  
  1603  	// instance expression
  1604  	return typeparams.PackIndexExpr(x, lbrack, args, rbrack)
  1605  }
  1606  
  1607  func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
  1608  	if p.trace {
  1609  		defer un(trace(p, "CallOrConversion"))
  1610  	}
  1611  
  1612  	lparen := p.expect(token.LPAREN)
  1613  	p.exprLev++
  1614  	var list []ast.Expr
  1615  	var ellipsis token.Pos
  1616  	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
  1617  		list = append(list, p.parseRhs()) // builtins may expect a type: make(some type, ...)
  1618  		if p.tok == token.ELLIPSIS {
  1619  			ellipsis = p.pos
  1620  			p.next()
  1621  		}
  1622  		if !p.atComma("argument list", token.RPAREN) {
  1623  			break
  1624  		}
  1625  		p.next()
  1626  	}
  1627  	p.exprLev--
  1628  	rparen := p.expectClosing(token.RPAREN, "argument list")
  1629  
  1630  	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
  1631  }
  1632  
  1633  func (p *parser) parseValue() ast.Expr {
  1634  	if p.trace {
  1635  		defer un(trace(p, "Element"))
  1636  	}
  1637  
  1638  	if p.tok == token.LBRACE {
  1639  		return p.parseLiteralValue(nil)
  1640  	}
  1641  
  1642  	x := p.parseExpr()
  1643  
  1644  	return x
  1645  }
  1646  
  1647  func (p *parser) parseElement() ast.Expr {
  1648  	if p.trace {
  1649  		defer un(trace(p, "Element"))
  1650  	}
  1651  
  1652  	x := p.parseValue()
  1653  	if p.tok == token.COLON {
  1654  		colon := p.pos
  1655  		p.next()
  1656  		x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue()}
  1657  	}
  1658  
  1659  	return x
  1660  }
  1661  
  1662  func (p *parser) parseElementList() (list []ast.Expr) {
  1663  	if p.trace {
  1664  		defer un(trace(p, "ElementList"))
  1665  	}
  1666  
  1667  	for p.tok != token.RBRACE && p.tok != token.EOF {
  1668  		list = append(list, p.parseElement())
  1669  		if !p.atComma("composite literal", token.RBRACE) {
  1670  			break
  1671  		}
  1672  		p.next()
  1673  	}
  1674  
  1675  	return
  1676  }
  1677  
  1678  func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
  1679  	if p.trace {
  1680  		defer un(trace(p, "LiteralValue"))
  1681  	}
  1682  
  1683  	lbrace := p.expect(token.LBRACE)
  1684  	var elts []ast.Expr
  1685  	p.exprLev++
  1686  	if p.tok != token.RBRACE {
  1687  		elts = p.parseElementList()
  1688  	}
  1689  	p.exprLev--
  1690  	rbrace := p.expectClosing(token.RBRACE, "composite literal")
  1691  	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
  1692  }
  1693  
  1694  func (p *parser) parsePrimaryExpr(x ast.Expr) ast.Expr {
  1695  	if p.trace {
  1696  		defer un(trace(p, "PrimaryExpr"))
  1697  	}
  1698  
  1699  	if x == nil {
  1700  		x = p.parseOperand()
  1701  	}
  1702  	// We track the nesting here rather than at the entry for the function,
  1703  	// since it can iteratively produce a nested output, and we want to
  1704  	// limit how deep a structure we generate.
  1705  	var n int
  1706  	defer func() { p.nestLev -= n }()
  1707  	for n = 1; ; n++ {
  1708  		incNestLev(p)
  1709  		switch p.tok {
  1710  		case token.PERIOD:
  1711  			p.next()
  1712  			switch p.tok {
  1713  			case token.IDENT:
  1714  				x = p.parseSelector(x)
  1715  			case token.LPAREN:
  1716  				x = p.parseTypeAssertion(x)
  1717  			default:
  1718  				pos := p.pos
  1719  				p.errorExpected(pos, "selector or type assertion")
  1720  				// TODO(rFindley) The check for token.RBRACE below is a targeted fix
  1721  				//                to error recovery sufficient to make the x/tools tests to
  1722  				//                pass with the new parsing logic introduced for type
  1723  				//                parameters. Remove this once error recovery has been
  1724  				//                more generally reconsidered.
  1725  				if p.tok != token.RBRACE {
  1726  					p.next() // make progress
  1727  				}
  1728  				sel := &ast.Ident{NamePos: pos, Name: "_"}
  1729  				x = &ast.SelectorExpr{X: x, Sel: sel}
  1730  			}
  1731  		case token.LBRACK:
  1732  			x = p.parseIndexOrSliceOrInstance(x)
  1733  		case token.LPAREN:
  1734  			x = p.parseCallOrConversion(x)
  1735  		case token.LBRACE:
  1736  			// operand may have returned a parenthesized complit
  1737  			// type; accept it but complain if we have a complit
  1738  			t := ast.Unparen(x)
  1739  			// determine if '{' belongs to a composite literal or a block statement
  1740  			switch t.(type) {
  1741  			case *ast.BadExpr, *ast.Ident, *ast.SelectorExpr:
  1742  				if p.exprLev < 0 {
  1743  					return x
  1744  				}
  1745  				// x is possibly a composite literal type
  1746  			case *ast.IndexExpr, *ast.IndexListExpr:
  1747  				if p.exprLev < 0 {
  1748  					return x
  1749  				}
  1750  				// x is possibly a composite literal type
  1751  			case *ast.ArrayType, *ast.StructType, *ast.MapType:
  1752  				// x is a composite literal type
  1753  			default:
  1754  				return x
  1755  			}
  1756  			if t != x {
  1757  				p.error(t.Pos(), "cannot parenthesize type in composite literal")
  1758  				// already progressed, no need to advance
  1759  			}
  1760  			x = p.parseLiteralValue(x)
  1761  		default:
  1762  			return x
  1763  		}
  1764  	}
  1765  }
  1766  
  1767  func (p *parser) parseUnaryExpr() ast.Expr {
  1768  	defer decNestLev(incNestLev(p))
  1769  
  1770  	if p.trace {
  1771  		defer un(trace(p, "UnaryExpr"))
  1772  	}
  1773  
  1774  	switch p.tok {
  1775  	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND, token.TILDE:
  1776  		pos, op := p.pos, p.tok
  1777  		p.next()
  1778  		x := p.parseUnaryExpr()
  1779  		return &ast.UnaryExpr{OpPos: pos, Op: op, X: x}
  1780  
  1781  	case token.ARROW:
  1782  		// channel type or receive expression
  1783  		arrow := p.pos
  1784  		p.next()
  1785  
  1786  		// If the next token is token.CHAN we still don't know if it
  1787  		// is a channel type or a receive operation - we only know
  1788  		// once we have found the end of the unary expression. There
  1789  		// are two cases:
  1790  		//
  1791  		//   <- type  => (<-type) must be channel type
  1792  		//   <- expr  => <-(expr) is a receive from an expression
  1793  		//
  1794  		// In the first case, the arrow must be re-associated with
  1795  		// the channel type parsed already:
  1796  		//
  1797  		//   <- (chan type)    =>  (<-chan type)
  1798  		//   <- (chan<- type)  =>  (<-chan (<-type))
  1799  
  1800  		x := p.parseUnaryExpr()
  1801  
  1802  		// determine which case we have
  1803  		if typ, ok := x.(*ast.ChanType); ok {
  1804  			// (<-type)
  1805  
  1806  			// re-associate position info and <-
  1807  			dir := ast.SEND
  1808  			for ok && dir == ast.SEND {
  1809  				if typ.Dir == ast.RECV {
  1810  					// error: (<-type) is (<-(<-chan T))
  1811  					p.errorExpected(typ.Arrow, "'chan'")
  1812  				}
  1813  				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
  1814  				dir, typ.Dir = typ.Dir, ast.RECV
  1815  				typ, ok = typ.Value.(*ast.ChanType)
  1816  			}
  1817  			if dir == ast.SEND {
  1818  				p.errorExpected(arrow, "channel type")
  1819  			}
  1820  
  1821  			return x
  1822  		}
  1823  
  1824  		// <-(expr)
  1825  		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: x}
  1826  
  1827  	case token.MUL:
  1828  		// pointer type or unary "*" expression
  1829  		pos := p.pos
  1830  		p.next()
  1831  		x := p.parseUnaryExpr()
  1832  		return &ast.StarExpr{Star: pos, X: x}
  1833  	}
  1834  
  1835  	return p.parsePrimaryExpr(nil)
  1836  }
  1837  
  1838  func (p *parser) tokPrec() (token.Token, int) {
  1839  	tok := p.tok
  1840  	if p.inRhs && tok == token.ASSIGN {
  1841  		tok = token.EQL
  1842  	}
  1843  	return tok, tok.Precedence()
  1844  }
  1845  
  1846  // parseBinaryExpr parses a (possibly) binary expression.
  1847  // If x is non-nil, it is used as the left operand.
  1848  //
  1849  // TODO(rfindley): parseBinaryExpr has become overloaded. Consider refactoring.
  1850  func (p *parser) parseBinaryExpr(x ast.Expr, prec1 int) ast.Expr {
  1851  	if p.trace {
  1852  		defer un(trace(p, "BinaryExpr"))
  1853  	}
  1854  
  1855  	if x == nil {
  1856  		x = p.parseUnaryExpr()
  1857  	}
  1858  	// We track the nesting here rather than at the entry for the function,
  1859  	// since it can iteratively produce a nested output, and we want to
  1860  	// limit how deep a structure we generate.
  1861  	var n int
  1862  	defer func() { p.nestLev -= n }()
  1863  	for n = 1; ; n++ {
  1864  		incNestLev(p)
  1865  		op, oprec := p.tokPrec()
  1866  		if oprec < prec1 {
  1867  			return x
  1868  		}
  1869  		pos := p.expect(op)
  1870  		y := p.parseBinaryExpr(nil, oprec+1)
  1871  		x = &ast.BinaryExpr{X: x, OpPos: pos, Op: op, Y: y}
  1872  	}
  1873  }
  1874  
  1875  // The result may be a type or even a raw type ([...]int).
  1876  func (p *parser) parseExpr() ast.Expr {
  1877  	if p.trace {
  1878  		defer un(trace(p, "Expression"))
  1879  	}
  1880  
  1881  	return p.parseBinaryExpr(nil, token.LowestPrec+1)
  1882  }
  1883  
  1884  func (p *parser) parseRhs() ast.Expr {
  1885  	old := p.inRhs
  1886  	p.inRhs = true
  1887  	x := p.parseExpr()
  1888  	p.inRhs = old
  1889  	return x
  1890  }
  1891  
  1892  // ----------------------------------------------------------------------------
  1893  // Statements
  1894  
  1895  // Parsing modes for parseSimpleStmt.
  1896  const (
  1897  	basic = iota
  1898  	labelOk
  1899  	rangeOk
  1900  )
  1901  
  1902  // parseSimpleStmt returns true as 2nd result if it parsed the assignment
  1903  // of a range clause (with mode == rangeOk). The returned statement is an
  1904  // assignment with a right-hand side that is a single unary expression of
  1905  // the form "range x". No guarantees are given for the left-hand side.
  1906  func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
  1907  	if p.trace {
  1908  		defer un(trace(p, "SimpleStmt"))
  1909  	}
  1910  
  1911  	x := p.parseList(false)
  1912  
  1913  	switch p.tok {
  1914  	case
  1915  		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
  1916  		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
  1917  		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
  1918  		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
  1919  		// assignment statement, possibly part of a range clause
  1920  		pos, tok := p.pos, p.tok
  1921  		p.next()
  1922  		var y []ast.Expr
  1923  		isRange := false
  1924  		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
  1925  			pos := p.pos
  1926  			p.next()
  1927  			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  1928  			isRange = true
  1929  		} else {
  1930  			y = p.parseList(true)
  1931  		}
  1932  		return &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}, isRange
  1933  	}
  1934  
  1935  	if len(x) > 1 {
  1936  		p.errorExpected(x[0].Pos(), "1 expression")
  1937  		// continue with first expression
  1938  	}
  1939  
  1940  	switch p.tok {
  1941  	case token.COLON:
  1942  		// labeled statement
  1943  		colon := p.pos
  1944  		p.next()
  1945  		if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
  1946  			// Go spec: The scope of a label is the body of the function
  1947  			// in which it is declared and excludes the body of any nested
  1948  			// function.
  1949  			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
  1950  			return stmt, false
  1951  		}
  1952  		// The label declaration typically starts at x[0].Pos(), but the label
  1953  		// declaration may be erroneous due to a token after that position (and
  1954  		// before the ':'). If SpuriousErrors is not set, the (only) error
  1955  		// reported for the line is the illegal label error instead of the token
  1956  		// before the ':' that caused the problem. Thus, use the (latest) colon
  1957  		// position for error reporting.
  1958  		p.error(colon, "illegal label declaration")
  1959  		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false
  1960  
  1961  	case token.ARROW:
  1962  		// send statement
  1963  		arrow := p.pos
  1964  		p.next()
  1965  		y := p.parseRhs()
  1966  		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false
  1967  
  1968  	case token.INC, token.DEC:
  1969  		// increment or decrement
  1970  		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
  1971  		p.next()
  1972  		return s, false
  1973  	}
  1974  
  1975  	// expression
  1976  	return &ast.ExprStmt{X: x[0]}, false
  1977  }
  1978  
  1979  func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
  1980  	x := p.parseRhs() // could be a conversion: (some type)(x)
  1981  	if t := ast.Unparen(x); t != x {
  1982  		p.error(x.Pos(), fmt.Sprintf("expression in %s must not be parenthesized", callType))
  1983  		x = t
  1984  	}
  1985  	if call, isCall := x.(*ast.CallExpr); isCall {
  1986  		return call
  1987  	}
  1988  	if _, isBad := x.(*ast.BadExpr); !isBad {
  1989  		// only report error if it's a new one
  1990  		p.error(p.safePos(x.End()), fmt.Sprintf("expression in %s must be function call", callType))
  1991  	}
  1992  	return nil
  1993  }
  1994  
  1995  func (p *parser) parseGoStmt() ast.Stmt {
  1996  	if p.trace {
  1997  		defer un(trace(p, "GoStmt"))
  1998  	}
  1999  
  2000  	pos := p.expect(token.GO)
  2001  	call := p.parseCallExpr("go")
  2002  	p.expectSemi()
  2003  	if call == nil {
  2004  		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
  2005  	}
  2006  
  2007  	return &ast.GoStmt{Go: pos, Call: call}
  2008  }
  2009  
  2010  func (p *parser) parseDeferStmt() ast.Stmt {
  2011  	if p.trace {
  2012  		defer un(trace(p, "DeferStmt"))
  2013  	}
  2014  
  2015  	pos := p.expect(token.DEFER)
  2016  	call := p.parseCallExpr("defer")
  2017  	p.expectSemi()
  2018  	if call == nil {
  2019  		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
  2020  	}
  2021  
  2022  	return &ast.DeferStmt{Defer: pos, Call: call}
  2023  }
  2024  
  2025  func (p *parser) parseReturnStmt() *ast.ReturnStmt {
  2026  	if p.trace {
  2027  		defer un(trace(p, "ReturnStmt"))
  2028  	}
  2029  
  2030  	pos := p.pos
  2031  	p.expect(token.RETURN)
  2032  	var x []ast.Expr
  2033  	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
  2034  		x = p.parseList(true)
  2035  	}
  2036  	p.expectSemi()
  2037  
  2038  	return &ast.ReturnStmt{Return: pos, Results: x}
  2039  }
  2040  
  2041  func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
  2042  	if p.trace {
  2043  		defer un(trace(p, "BranchStmt"))
  2044  	}
  2045  
  2046  	pos := p.expect(tok)
  2047  	var label *ast.Ident
  2048  	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
  2049  		label = p.parseIdent()
  2050  	}
  2051  	p.expectSemi()
  2052  
  2053  	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
  2054  }
  2055  
  2056  func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
  2057  	if s == nil {
  2058  		return nil
  2059  	}
  2060  	if es, isExpr := s.(*ast.ExprStmt); isExpr {
  2061  		return es.X
  2062  	}
  2063  	found := "simple statement"
  2064  	if _, isAss := s.(*ast.AssignStmt); isAss {
  2065  		found = "assignment"
  2066  	}
  2067  	p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
  2068  	return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
  2069  }
  2070  
  2071  // parseIfHeader is an adjusted version of parser.header
  2072  // in cmd/compile/internal/syntax/parser.go, which has
  2073  // been tuned for better error handling.
  2074  func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
  2075  	if p.tok == token.LBRACE {
  2076  		p.error(p.pos, "missing condition in if statement")
  2077  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  2078  		return
  2079  	}
  2080  	// p.tok != token.LBRACE
  2081  
  2082  	prevLev := p.exprLev
  2083  	p.exprLev = -1
  2084  
  2085  	if p.tok != token.SEMICOLON {
  2086  		// accept potential variable declaration but complain
  2087  		if p.tok == token.VAR {
  2088  			p.next()
  2089  			p.error(p.pos, "var declaration not allowed in if initializer")
  2090  		}
  2091  		init, _ = p.parseSimpleStmt(basic)
  2092  	}
  2093  
  2094  	var condStmt ast.Stmt
  2095  	var semi struct {
  2096  		pos token.Pos
  2097  		lit string // ";" or "\n"; valid if pos.IsValid()
  2098  	}
  2099  	if p.tok != token.LBRACE {
  2100  		if p.tok == token.SEMICOLON {
  2101  			semi.pos = p.pos
  2102  			semi.lit = p.lit
  2103  			p.next()
  2104  		} else {
  2105  			p.expect(token.SEMICOLON)
  2106  		}
  2107  		if p.tok != token.LBRACE {
  2108  			condStmt, _ = p.parseSimpleStmt(basic)
  2109  		}
  2110  	} else {
  2111  		condStmt = init
  2112  		init = nil
  2113  	}
  2114  
  2115  	if condStmt != nil {
  2116  		cond = p.makeExpr(condStmt, "boolean expression")
  2117  	} else if semi.pos.IsValid() {
  2118  		if semi.lit == "\n" {
  2119  			p.error(semi.pos, "unexpected newline, expecting { after if clause")
  2120  		} else {
  2121  			p.error(semi.pos, "missing condition in if statement")
  2122  		}
  2123  	}
  2124  
  2125  	// make sure we have a valid AST
  2126  	if cond == nil {
  2127  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  2128  	}
  2129  
  2130  	p.exprLev = prevLev
  2131  	return
  2132  }
  2133  
  2134  func (p *parser) parseIfStmt() *ast.IfStmt {
  2135  	defer decNestLev(incNestLev(p))
  2136  
  2137  	if p.trace {
  2138  		defer un(trace(p, "IfStmt"))
  2139  	}
  2140  
  2141  	pos := p.expect(token.IF)
  2142  
  2143  	init, cond := p.parseIfHeader()
  2144  	body := p.parseBlockStmt()
  2145  
  2146  	var else_ ast.Stmt
  2147  	if p.tok == token.ELSE {
  2148  		p.next()
  2149  		switch p.tok {
  2150  		case token.IF:
  2151  			else_ = p.parseIfStmt()
  2152  		case token.LBRACE:
  2153  			else_ = p.parseBlockStmt()
  2154  			p.expectSemi()
  2155  		default:
  2156  			p.errorExpected(p.pos, "if statement or block")
  2157  			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
  2158  		}
  2159  	} else {
  2160  		p.expectSemi()
  2161  	}
  2162  
  2163  	return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
  2164  }
  2165  
  2166  func (p *parser) parseCaseClause() *ast.CaseClause {
  2167  	if p.trace {
  2168  		defer un(trace(p, "CaseClause"))
  2169  	}
  2170  
  2171  	pos := p.pos
  2172  	var list []ast.Expr
  2173  	if p.tok == token.CASE {
  2174  		p.next()
  2175  		list = p.parseList(true)
  2176  	} else {
  2177  		p.expect(token.DEFAULT)
  2178  	}
  2179  
  2180  	colon := p.expect(token.COLON)
  2181  	body := p.parseStmtList()
  2182  
  2183  	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
  2184  }
  2185  
  2186  func isTypeSwitchAssert(x ast.Expr) bool {
  2187  	a, ok := x.(*ast.TypeAssertExpr)
  2188  	return ok && a.Type == nil
  2189  }
  2190  
  2191  func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
  2192  	switch t := s.(type) {
  2193  	case *ast.ExprStmt:
  2194  		// x.(type)
  2195  		return isTypeSwitchAssert(t.X)
  2196  	case *ast.AssignStmt:
  2197  		// v := x.(type)
  2198  		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
  2199  			switch t.Tok {
  2200  			case token.ASSIGN:
  2201  				// permit v = x.(type) but complain
  2202  				p.error(t.TokPos, "expected ':=', found '='")
  2203  				fallthrough
  2204  			case token.DEFINE:
  2205  				return true
  2206  			}
  2207  		}
  2208  	}
  2209  	return false
  2210  }
  2211  
  2212  func (p *parser) parseSwitchStmt() ast.Stmt {
  2213  	if p.trace {
  2214  		defer un(trace(p, "SwitchStmt"))
  2215  	}
  2216  
  2217  	pos := p.expect(token.SWITCH)
  2218  
  2219  	var s1, s2 ast.Stmt
  2220  	if p.tok != token.LBRACE {
  2221  		prevLev := p.exprLev
  2222  		p.exprLev = -1
  2223  		if p.tok != token.SEMICOLON {
  2224  			s2, _ = p.parseSimpleStmt(basic)
  2225  		}
  2226  		if p.tok == token.SEMICOLON {
  2227  			p.next()
  2228  			s1 = s2
  2229  			s2 = nil
  2230  			if p.tok != token.LBRACE {
  2231  				// A TypeSwitchGuard may declare a variable in addition
  2232  				// to the variable declared in the initial SimpleStmt.
  2233  				// Introduce extra scope to avoid redeclaration errors:
  2234  				//
  2235  				//	switch t := 0; t := x.(T) { ... }
  2236  				//
  2237  				// (this code is not valid Go because the first t
  2238  				// cannot be accessed and thus is never used, the extra
  2239  				// scope is needed for the correct error message).
  2240  				//
  2241  				// If we don't have a type switch, s2 must be an expression.
  2242  				// Having the extra nested but empty scope won't affect it.
  2243  				s2, _ = p.parseSimpleStmt(basic)
  2244  			}
  2245  		}
  2246  		p.exprLev = prevLev
  2247  	}
  2248  
  2249  	typeSwitch := p.isTypeSwitchGuard(s2)
  2250  	lbrace := p.expect(token.LBRACE)
  2251  	var list []ast.Stmt
  2252  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2253  		list = append(list, p.parseCaseClause())
  2254  	}
  2255  	rbrace := p.expect(token.RBRACE)
  2256  	p.expectSemi()
  2257  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2258  
  2259  	if typeSwitch {
  2260  		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
  2261  	}
  2262  
  2263  	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
  2264  }
  2265  
  2266  func (p *parser) parseCommClause() *ast.CommClause {
  2267  	if p.trace {
  2268  		defer un(trace(p, "CommClause"))
  2269  	}
  2270  
  2271  	pos := p.pos
  2272  	var comm ast.Stmt
  2273  	if p.tok == token.CASE {
  2274  		p.next()
  2275  		lhs := p.parseList(false)
  2276  		if p.tok == token.ARROW {
  2277  			// SendStmt
  2278  			if len(lhs) > 1 {
  2279  				p.errorExpected(lhs[0].Pos(), "1 expression")
  2280  				// continue with first expression
  2281  			}
  2282  			arrow := p.pos
  2283  			p.next()
  2284  			rhs := p.parseRhs()
  2285  			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
  2286  		} else {
  2287  			// RecvStmt
  2288  			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
  2289  				// RecvStmt with assignment
  2290  				if len(lhs) > 2 {
  2291  					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
  2292  					// continue with first two expressions
  2293  					lhs = lhs[0:2]
  2294  				}
  2295  				pos := p.pos
  2296  				p.next()
  2297  				rhs := p.parseRhs()
  2298  				comm = &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
  2299  			} else {
  2300  				// lhs must be single receive operation
  2301  				if len(lhs) > 1 {
  2302  					p.errorExpected(lhs[0].Pos(), "1 expression")
  2303  					// continue with first expression
  2304  				}
  2305  				comm = &ast.ExprStmt{X: lhs[0]}
  2306  			}
  2307  		}
  2308  	} else {
  2309  		p.expect(token.DEFAULT)
  2310  	}
  2311  
  2312  	colon := p.expect(token.COLON)
  2313  	body := p.parseStmtList()
  2314  
  2315  	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
  2316  }
  2317  
  2318  func (p *parser) parseSelectStmt() *ast.SelectStmt {
  2319  	if p.trace {
  2320  		defer un(trace(p, "SelectStmt"))
  2321  	}
  2322  
  2323  	pos := p.expect(token.SELECT)
  2324  	lbrace := p.expect(token.LBRACE)
  2325  	var list []ast.Stmt
  2326  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2327  		list = append(list, p.parseCommClause())
  2328  	}
  2329  	rbrace := p.expect(token.RBRACE)
  2330  	p.expectSemi()
  2331  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2332  
  2333  	return &ast.SelectStmt{Select: pos, Body: body}
  2334  }
  2335  
  2336  func (p *parser) parseForStmt() ast.Stmt {
  2337  	if p.trace {
  2338  		defer un(trace(p, "ForStmt"))
  2339  	}
  2340  
  2341  	pos := p.expect(token.FOR)
  2342  
  2343  	var s1, s2, s3 ast.Stmt
  2344  	var isRange bool
  2345  	if p.tok != token.LBRACE {
  2346  		prevLev := p.exprLev
  2347  		p.exprLev = -1
  2348  		if p.tok != token.SEMICOLON {
  2349  			if p.tok == token.RANGE {
  2350  				// "for range x" (nil lhs in assignment)
  2351  				pos := p.pos
  2352  				p.next()
  2353  				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  2354  				s2 = &ast.AssignStmt{Rhs: y}
  2355  				isRange = true
  2356  			} else {
  2357  				s2, isRange = p.parseSimpleStmt(rangeOk)
  2358  			}
  2359  		}
  2360  		if !isRange && p.tok == token.SEMICOLON {
  2361  			p.next()
  2362  			s1 = s2
  2363  			s2 = nil
  2364  			if p.tok != token.SEMICOLON {
  2365  				s2, _ = p.parseSimpleStmt(basic)
  2366  			}
  2367  			p.expectSemi()
  2368  			if p.tok != token.LBRACE {
  2369  				s3, _ = p.parseSimpleStmt(basic)
  2370  			}
  2371  		}
  2372  		p.exprLev = prevLev
  2373  	}
  2374  
  2375  	body := p.parseBlockStmt()
  2376  	p.expectSemi()
  2377  
  2378  	if isRange {
  2379  		as := s2.(*ast.AssignStmt)
  2380  		// check lhs
  2381  		var key, value ast.Expr
  2382  		switch len(as.Lhs) {
  2383  		case 0:
  2384  			// nothing to do
  2385  		case 1:
  2386  			key = as.Lhs[0]
  2387  		case 2:
  2388  			key, value = as.Lhs[0], as.Lhs[1]
  2389  		default:
  2390  			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
  2391  			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
  2392  		}
  2393  		// parseSimpleStmt returned a right-hand side that
  2394  		// is a single unary expression of the form "range x"
  2395  		x := as.Rhs[0].(*ast.UnaryExpr).X
  2396  		return &ast.RangeStmt{
  2397  			For:    pos,
  2398  			Key:    key,
  2399  			Value:  value,
  2400  			TokPos: as.TokPos,
  2401  			Tok:    as.Tok,
  2402  			Range:  as.Rhs[0].Pos(),
  2403  			X:      x,
  2404  			Body:   body,
  2405  		}
  2406  	}
  2407  
  2408  	// regular for statement
  2409  	return &ast.ForStmt{
  2410  		For:  pos,
  2411  		Init: s1,
  2412  		Cond: p.makeExpr(s2, "boolean or range expression"),
  2413  		Post: s3,
  2414  		Body: body,
  2415  	}
  2416  }
  2417  
  2418  func (p *parser) parseStmt() (s ast.Stmt) {
  2419  	defer decNestLev(incNestLev(p))
  2420  
  2421  	if p.trace {
  2422  		defer un(trace(p, "Statement"))
  2423  	}
  2424  
  2425  	switch p.tok {
  2426  	case token.CONST, token.TYPE, token.VAR:
  2427  		s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}
  2428  	case
  2429  		// tokens that may start an expression
  2430  		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
  2431  		token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
  2432  		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
  2433  		s, _ = p.parseSimpleStmt(labelOk)
  2434  		// because of the required look-ahead, labeled statements are
  2435  		// parsed by parseSimpleStmt - don't expect a semicolon after
  2436  		// them
  2437  		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
  2438  			p.expectSemi()
  2439  		}
  2440  	case token.GO:
  2441  		s = p.parseGoStmt()
  2442  	case token.DEFER:
  2443  		s = p.parseDeferStmt()
  2444  	case token.RETURN:
  2445  		s = p.parseReturnStmt()
  2446  	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
  2447  		s = p.parseBranchStmt(p.tok)
  2448  	case token.LBRACE:
  2449  		s = p.parseBlockStmt()
  2450  		p.expectSemi()
  2451  	case token.IF:
  2452  		s = p.parseIfStmt()
  2453  	case token.SWITCH:
  2454  		s = p.parseSwitchStmt()
  2455  	case token.SELECT:
  2456  		s = p.parseSelectStmt()
  2457  	case token.FOR:
  2458  		s = p.parseForStmt()
  2459  	case token.SEMICOLON:
  2460  		// Is it ever possible to have an implicit semicolon
  2461  		// producing an empty statement in a valid program?
  2462  		// (handle correctly anyway)
  2463  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
  2464  		p.next()
  2465  	case token.RBRACE:
  2466  		// a semicolon may be omitted before a closing "}"
  2467  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}
  2468  	default:
  2469  		// no statement found
  2470  		pos := p.pos
  2471  		p.errorExpected(pos, "statement")
  2472  		p.advance(stmtStart)
  2473  		s = &ast.BadStmt{From: pos, To: p.pos}
  2474  	}
  2475  
  2476  	return
  2477  }
  2478  
  2479  // ----------------------------------------------------------------------------
  2480  // Declarations
  2481  
  2482  type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec
  2483  
  2484  func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2485  	if p.trace {
  2486  		defer un(trace(p, "ImportSpec"))
  2487  	}
  2488  
  2489  	var ident *ast.Ident
  2490  	switch p.tok {
  2491  	case token.IDENT:
  2492  		ident = p.parseIdent()
  2493  	case token.PERIOD:
  2494  		ident = &ast.Ident{NamePos: p.pos, Name: "."}
  2495  		p.next()
  2496  	}
  2497  
  2498  	pos := p.pos
  2499  	var path string
  2500  	if p.tok == token.STRING {
  2501  		path = p.lit
  2502  		p.next()
  2503  	} else if p.tok.IsLiteral() {
  2504  		p.error(pos, "import path must be a string")
  2505  		p.next()
  2506  	} else {
  2507  		p.error(pos, "missing import path")
  2508  		p.advance(exprEnd)
  2509  	}
  2510  	comment := p.expectSemi()
  2511  
  2512  	// collect imports
  2513  	spec := &ast.ImportSpec{
  2514  		Doc:     doc,
  2515  		Name:    ident,
  2516  		Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
  2517  		Comment: comment,
  2518  	}
  2519  	p.imports = append(p.imports, spec)
  2520  
  2521  	return spec
  2522  }
  2523  
  2524  func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
  2525  	if p.trace {
  2526  		defer un(trace(p, keyword.String()+"Spec"))
  2527  	}
  2528  
  2529  	idents := p.parseIdentList()
  2530  	var typ ast.Expr
  2531  	var values []ast.Expr
  2532  	switch keyword {
  2533  	case token.CONST:
  2534  		// always permit optional type and initialization for more tolerant parsing
  2535  		if p.tok != token.EOF && p.tok != token.SEMICOLON && p.tok != token.RPAREN {
  2536  			typ = p.tryIdentOrType()
  2537  			if p.tok == token.ASSIGN {
  2538  				p.next()
  2539  				values = p.parseList(true)
  2540  			}
  2541  		}
  2542  	case token.VAR:
  2543  		if p.tok != token.ASSIGN {
  2544  			typ = p.parseType()
  2545  		}
  2546  		if p.tok == token.ASSIGN {
  2547  			p.next()
  2548  			values = p.parseList(true)
  2549  		}
  2550  	default:
  2551  		panic("unreachable")
  2552  	}
  2553  	comment := p.expectSemi()
  2554  
  2555  	spec := &ast.ValueSpec{
  2556  		Doc:     doc,
  2557  		Names:   idents,
  2558  		Type:    typ,
  2559  		Values:  values,
  2560  		Comment: comment,
  2561  	}
  2562  	return spec
  2563  }
  2564  
  2565  func (p *parser) parseGenericType(spec *ast.TypeSpec, openPos token.Pos, name0 *ast.Ident, typ0 ast.Expr) {
  2566  	if p.trace {
  2567  		defer un(trace(p, "parseGenericType"))
  2568  	}
  2569  
  2570  	list := p.parseParameterList(name0, typ0, token.RBRACK)
  2571  	closePos := p.expect(token.RBRACK)
  2572  	spec.TypeParams = &ast.FieldList{Opening: openPos, List: list, Closing: closePos}
  2573  	// Let the type checker decide whether to accept type parameters on aliases:
  2574  	// see go.dev/issue/46477.
  2575  	if p.tok == token.ASSIGN {
  2576  		// type alias
  2577  		spec.Assign = p.pos
  2578  		p.next()
  2579  	}
  2580  	spec.Type = p.parseType()
  2581  }
  2582  
  2583  func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2584  	if p.trace {
  2585  		defer un(trace(p, "TypeSpec"))
  2586  	}
  2587  
  2588  	name := p.parseIdent()
  2589  	spec := &ast.TypeSpec{Doc: doc, Name: name}
  2590  
  2591  	if p.tok == token.LBRACK {
  2592  		// spec.Name "[" ...
  2593  		// array/slice type or type parameter list
  2594  		lbrack := p.pos
  2595  		p.next()
  2596  		if p.tok == token.IDENT {
  2597  			// We may have an array type or a type parameter list.
  2598  			// In either case we expect an expression x (which may
  2599  			// just be a name, or a more complex expression) which
  2600  			// we can analyze further.
  2601  			//
  2602  			// A type parameter list may have a type bound starting
  2603  			// with a "[" as in: P []E. In that case, simply parsing
  2604  			// an expression would lead to an error: P[] is invalid.
  2605  			// But since index or slice expressions are never constant
  2606  			// and thus invalid array length expressions, if the name
  2607  			// is followed by "[" it must be the start of an array or
  2608  			// slice constraint. Only if we don't see a "[" do we
  2609  			// need to parse a full expression. Notably, name <- x
  2610  			// is not a concern because name <- x is a statement and
  2611  			// not an expression.
  2612  			var x ast.Expr = p.parseIdent()
  2613  			if p.tok != token.LBRACK {
  2614  				// To parse the expression starting with name, expand
  2615  				// the call sequence we would get by passing in name
  2616  				// to parser.expr, and pass in name to parsePrimaryExpr.
  2617  				p.exprLev++
  2618  				lhs := p.parsePrimaryExpr(x)
  2619  				x = p.parseBinaryExpr(lhs, token.LowestPrec+1)
  2620  				p.exprLev--
  2621  			}
  2622  			// Analyze expression x. If we can split x into a type parameter
  2623  			// name, possibly followed by a type parameter type, we consider
  2624  			// this the start of a type parameter list, with some caveats:
  2625  			// a single name followed by "]" tilts the decision towards an
  2626  			// array declaration; a type parameter type that could also be
  2627  			// an ordinary expression but which is followed by a comma tilts
  2628  			// the decision towards a type parameter list.
  2629  			if pname, ptype := extractName(x, p.tok == token.COMMA); pname != nil && (ptype != nil || p.tok != token.RBRACK) {
  2630  				// spec.Name "[" pname ...
  2631  				// spec.Name "[" pname ptype ...
  2632  				// spec.Name "[" pname ptype "," ...
  2633  				p.parseGenericType(spec, lbrack, pname, ptype) // ptype may be nil
  2634  			} else {
  2635  				// spec.Name "[" pname "]" ...
  2636  				// spec.Name "[" x ...
  2637  				spec.Type = p.parseArrayType(lbrack, x)
  2638  			}
  2639  		} else {
  2640  			// array type
  2641  			spec.Type = p.parseArrayType(lbrack, nil)
  2642  		}
  2643  	} else {
  2644  		// no type parameters
  2645  		if p.tok == token.ASSIGN {
  2646  			// type alias
  2647  			spec.Assign = p.pos
  2648  			p.next()
  2649  		}
  2650  		spec.Type = p.parseType()
  2651  	}
  2652  
  2653  	spec.Comment = p.expectSemi()
  2654  
  2655  	return spec
  2656  }
  2657  
  2658  // extractName splits the expression x into (name, expr) if syntactically
  2659  // x can be written as name expr. The split only happens if expr is a type
  2660  // element (per the isTypeElem predicate) or if force is set.
  2661  // If x is just a name, the result is (name, nil). If the split succeeds,
  2662  // the result is (name, expr). Otherwise the result is (nil, x).
  2663  // Examples:
  2664  //
  2665  //	x           force    name    expr
  2666  //	------------------------------------
  2667  //	P*[]int     T/F      P       *[]int
  2668  //	P*E         T        P       *E
  2669  //	P*E         F        nil     P*E
  2670  //	P([]int)    T/F      P       []int
  2671  //	P(E)        T        P       E
  2672  //	P(E)        F        nil     P(E)
  2673  //	P*E|F|~G    T/F      P       *E|F|~G
  2674  //	P*E|F|G     T        P       *E|F|G
  2675  //	P*E|F|G     F        nil     P*E|F|G
  2676  func extractName(x ast.Expr, force bool) (*ast.Ident, ast.Expr) {
  2677  	switch x := x.(type) {
  2678  	case *ast.Ident:
  2679  		return x, nil
  2680  	case *ast.BinaryExpr:
  2681  		switch x.Op {
  2682  		case token.MUL:
  2683  			if name, _ := x.X.(*ast.Ident); name != nil && (force || isTypeElem(x.Y)) {
  2684  				// x = name *x.Y
  2685  				return name, &ast.StarExpr{Star: x.OpPos, X: x.Y}
  2686  			}
  2687  		case token.OR:
  2688  			if name, lhs := extractName(x.X, force || isTypeElem(x.Y)); name != nil && lhs != nil {
  2689  				// x = name lhs|x.Y
  2690  				op := *x
  2691  				op.X = lhs
  2692  				return name, &op
  2693  			}
  2694  		}
  2695  	case *ast.CallExpr:
  2696  		if name, _ := x.Fun.(*ast.Ident); name != nil {
  2697  			if len(x.Args) == 1 && x.Ellipsis == token.NoPos && (force || isTypeElem(x.Args[0])) {
  2698  				// x = name "(" x.ArgList[0] ")"
  2699  				return name, x.Args[0]
  2700  			}
  2701  		}
  2702  	}
  2703  	return nil, x
  2704  }
  2705  
  2706  // isTypeElem reports whether x is a (possibly parenthesized) type element expression.
  2707  // The result is false if x could be a type element OR an ordinary (value) expression.
  2708  func isTypeElem(x ast.Expr) bool {
  2709  	switch x := x.(type) {
  2710  	case *ast.ArrayType, *ast.StructType, *ast.FuncType, *ast.InterfaceType, *ast.MapType, *ast.ChanType:
  2711  		return true
  2712  	case *ast.BinaryExpr:
  2713  		return isTypeElem(x.X) || isTypeElem(x.Y)
  2714  	case *ast.UnaryExpr:
  2715  		return x.Op == token.TILDE
  2716  	case *ast.ParenExpr:
  2717  		return isTypeElem(x.X)
  2718  	}
  2719  	return false
  2720  }
  2721  
  2722  func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
  2723  	if p.trace {
  2724  		defer un(trace(p, "GenDecl("+keyword.String()+")"))
  2725  	}
  2726  
  2727  	doc := p.leadComment
  2728  	pos := p.expect(keyword)
  2729  	var lparen, rparen token.Pos
  2730  	var list []ast.Spec
  2731  	if p.tok == token.LPAREN {
  2732  		lparen = p.pos
  2733  		p.next()
  2734  		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
  2735  			list = append(list, f(p.leadComment, keyword, iota))
  2736  		}
  2737  		rparen = p.expect(token.RPAREN)
  2738  		p.expectSemi()
  2739  	} else {
  2740  		list = append(list, f(nil, keyword, 0))
  2741  	}
  2742  
  2743  	return &ast.GenDecl{
  2744  		Doc:    doc,
  2745  		TokPos: pos,
  2746  		Tok:    keyword,
  2747  		Lparen: lparen,
  2748  		Specs:  list,
  2749  		Rparen: rparen,
  2750  	}
  2751  }
  2752  
  2753  func (p *parser) parseFuncDecl() *ast.FuncDecl {
  2754  	if p.trace {
  2755  		defer un(trace(p, "FunctionDecl"))
  2756  	}
  2757  
  2758  	doc := p.leadComment
  2759  	pos := p.expect(token.FUNC)
  2760  
  2761  	var recv *ast.FieldList
  2762  	if p.tok == token.LPAREN {
  2763  		_, recv = p.parseParameters(false)
  2764  	}
  2765  
  2766  	ident := p.parseIdent()
  2767  
  2768  	tparams, params := p.parseParameters(true)
  2769  	if recv != nil && tparams != nil {
  2770  		// Method declarations do not have type parameters. We parse them for a
  2771  		// better error message and improved error recovery.
  2772  		p.error(tparams.Opening, "method must have no type parameters")
  2773  		tparams = nil
  2774  	}
  2775  	results := p.parseResult()
  2776  
  2777  	var body *ast.BlockStmt
  2778  	switch p.tok {
  2779  	case token.LBRACE:
  2780  		body = p.parseBody()
  2781  		p.expectSemi()
  2782  	case token.SEMICOLON:
  2783  		p.next()
  2784  		if p.tok == token.LBRACE {
  2785  			// opening { of function declaration on next line
  2786  			p.error(p.pos, "unexpected semicolon or newline before {")
  2787  			body = p.parseBody()
  2788  			p.expectSemi()
  2789  		}
  2790  	default:
  2791  		p.expectSemi()
  2792  	}
  2793  
  2794  	decl := &ast.FuncDecl{
  2795  		Doc:  doc,
  2796  		Recv: recv,
  2797  		Name: ident,
  2798  		Type: &ast.FuncType{
  2799  			Func:       pos,
  2800  			TypeParams: tparams,
  2801  			Params:     params,
  2802  			Results:    results,
  2803  		},
  2804  		Body: body,
  2805  	}
  2806  	return decl
  2807  }
  2808  
  2809  func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
  2810  	if p.trace {
  2811  		defer un(trace(p, "Declaration"))
  2812  	}
  2813  
  2814  	var f parseSpecFunction
  2815  	switch p.tok {
  2816  	case token.IMPORT:
  2817  		f = p.parseImportSpec
  2818  
  2819  	case token.CONST, token.VAR:
  2820  		f = p.parseValueSpec
  2821  
  2822  	case token.TYPE:
  2823  		f = p.parseTypeSpec
  2824  
  2825  	case token.FUNC:
  2826  		return p.parseFuncDecl()
  2827  
  2828  	default:
  2829  		pos := p.pos
  2830  		p.errorExpected(pos, "declaration")
  2831  		p.advance(sync)
  2832  		return &ast.BadDecl{From: pos, To: p.pos}
  2833  	}
  2834  
  2835  	return p.parseGenDecl(p.tok, f)
  2836  }
  2837  
  2838  // ----------------------------------------------------------------------------
  2839  // Source files
  2840  
  2841  func (p *parser) parseFile() *ast.File {
  2842  	if p.trace {
  2843  		defer un(trace(p, "File"))
  2844  	}
  2845  
  2846  	// Don't bother parsing the rest if we had errors scanning the first token.
  2847  	// Likely not a Go source file at all.
  2848  	if p.errors.Len() != 0 {
  2849  		return nil
  2850  	}
  2851  
  2852  	// package clause
  2853  	doc := p.leadComment
  2854  	pos := p.expect(token.PACKAGE)
  2855  	// Go spec: The package clause is not a declaration;
  2856  	// the package name does not appear in any scope.
  2857  	ident := p.parseIdent()
  2858  	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
  2859  		p.error(p.pos, "invalid package name _")
  2860  	}
  2861  	p.expectSemi()
  2862  
  2863  	// Don't bother parsing the rest if we had errors parsing the package clause.
  2864  	// Likely not a Go source file at all.
  2865  	if p.errors.Len() != 0 {
  2866  		return nil
  2867  	}
  2868  
  2869  	var decls []ast.Decl
  2870  	if p.mode&PackageClauseOnly == 0 {
  2871  		// import decls
  2872  		for p.tok == token.IMPORT {
  2873  			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
  2874  		}
  2875  
  2876  		if p.mode&ImportsOnly == 0 {
  2877  			// rest of package body
  2878  			prev := token.IMPORT
  2879  			for p.tok != token.EOF {
  2880  				// Continue to accept import declarations for error tolerance, but complain.
  2881  				if p.tok == token.IMPORT && prev != token.IMPORT {
  2882  					p.error(p.pos, "imports must appear before other declarations")
  2883  				}
  2884  				prev = p.tok
  2885  
  2886  				decls = append(decls, p.parseDecl(declStart))
  2887  			}
  2888  		}
  2889  	}
  2890  
  2891  	f := &ast.File{
  2892  		Doc:       doc,
  2893  		Package:   pos,
  2894  		Name:      ident,
  2895  		Decls:     decls,
  2896  		FileStart: token.Pos(p.file.Base()),
  2897  		FileEnd:   token.Pos(p.file.Base() + p.file.Size()),
  2898  		Imports:   p.imports,
  2899  		Comments:  p.comments,
  2900  		GoVersion: p.goVersion,
  2901  	}
  2902  	var declErr func(token.Pos, string)
  2903  	if p.mode&DeclarationErrors != 0 {
  2904  		declErr = p.error
  2905  	}
  2906  	if p.mode&SkipObjectResolution == 0 {
  2907  		resolveFile(f, p.file, declErr)
  2908  	}
  2909  
  2910  	return f
  2911  }
  2912  

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