Source file src/go/types/stmt.go

Documentation: go/types

     1  // Copyright 2012 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  // This file implements typechecking of statements.
     6  
     7  package types
     8  
     9  import (
    10  	"go/ast"
    11  	"go/constant"
    12  	"go/token"
    13  	"internal/buildcfg"
    14  	. "internal/types/errors"
    15  	"sort"
    16  )
    17  
    18  func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body *ast.BlockStmt, iota constant.Value) {
    19  	if check.conf.IgnoreFuncBodies {
    20  		panic("function body not ignored")
    21  	}
    22  
    23  	if check.conf._Trace {
    24  		check.trace(body.Pos(), "-- %s: %s", name, sig)
    25  	}
    26  
    27  	// save/restore current environment and set up function environment
    28  	// (and use 0 indentation at function start)
    29  	defer func(env environment, indent int) {
    30  		check.environment = env
    31  		check.indent = indent
    32  	}(check.environment, check.indent)
    33  	check.environment = environment{
    34  		decl:  decl,
    35  		scope: sig.scope,
    36  		iota:  iota,
    37  		sig:   sig,
    38  	}
    39  	check.indent = 0
    40  
    41  	check.stmtList(0, body.List)
    42  
    43  	if check.hasLabel {
    44  		check.labels(body)
    45  	}
    46  
    47  	if sig.results.Len() > 0 && !check.isTerminating(body, "") {
    48  		check.error(atPos(body.Rbrace), MissingReturn, "missing return")
    49  	}
    50  
    51  	// spec: "Implementation restriction: A compiler may make it illegal to
    52  	// declare a variable inside a function body if the variable is never used."
    53  	check.usage(sig.scope)
    54  }
    55  
    56  func (check *Checker) usage(scope *Scope) {
    57  	var unused []*Var
    58  	for name, elem := range scope.elems {
    59  		elem = resolve(name, elem)
    60  		if v, _ := elem.(*Var); v != nil && !v.used {
    61  			unused = append(unused, v)
    62  		}
    63  	}
    64  	sort.Slice(unused, func(i, j int) bool {
    65  		return cmpPos(unused[i].pos, unused[j].pos) < 0
    66  	})
    67  	for _, v := range unused {
    68  		check.softErrorf(v, UnusedVar, "%s declared and not used", v.name)
    69  	}
    70  
    71  	for _, scope := range scope.children {
    72  		// Don't go inside function literal scopes a second time;
    73  		// they are handled explicitly by funcBody.
    74  		if !scope.isFunc {
    75  			check.usage(scope)
    76  		}
    77  	}
    78  }
    79  
    80  // stmtContext is a bitset describing which
    81  // control-flow statements are permissible,
    82  // and provides additional context information
    83  // for better error messages.
    84  type stmtContext uint
    85  
    86  const (
    87  	// permissible control-flow statements
    88  	breakOk stmtContext = 1 << iota
    89  	continueOk
    90  	fallthroughOk
    91  
    92  	// additional context information
    93  	finalSwitchCase
    94  	inTypeSwitch
    95  )
    96  
    97  func (check *Checker) simpleStmt(s ast.Stmt) {
    98  	if s != nil {
    99  		check.stmt(0, s)
   100  	}
   101  }
   102  
   103  func trimTrailingEmptyStmts(list []ast.Stmt) []ast.Stmt {
   104  	for i := len(list); i > 0; i-- {
   105  		if _, ok := list[i-1].(*ast.EmptyStmt); !ok {
   106  			return list[:i]
   107  		}
   108  	}
   109  	return nil
   110  }
   111  
   112  func (check *Checker) stmtList(ctxt stmtContext, list []ast.Stmt) {
   113  	ok := ctxt&fallthroughOk != 0
   114  	inner := ctxt &^ fallthroughOk
   115  	list = trimTrailingEmptyStmts(list) // trailing empty statements are "invisible" to fallthrough analysis
   116  	for i, s := range list {
   117  		inner := inner
   118  		if ok && i+1 == len(list) {
   119  			inner |= fallthroughOk
   120  		}
   121  		check.stmt(inner, s)
   122  	}
   123  }
   124  
   125  func (check *Checker) multipleDefaults(list []ast.Stmt) {
   126  	var first ast.Stmt
   127  	for _, s := range list {
   128  		var d ast.Stmt
   129  		switch c := s.(type) {
   130  		case *ast.CaseClause:
   131  			if len(c.List) == 0 {
   132  				d = s
   133  			}
   134  		case *ast.CommClause:
   135  			if c.Comm == nil {
   136  				d = s
   137  			}
   138  		default:
   139  			check.error(s, InvalidSyntaxTree, "case/communication clause expected")
   140  		}
   141  		if d != nil {
   142  			if first != nil {
   143  				check.errorf(d, DuplicateDefault, "multiple defaults (first at %s)", check.fset.Position(first.Pos()))
   144  			} else {
   145  				first = d
   146  			}
   147  		}
   148  	}
   149  }
   150  
   151  func (check *Checker) openScope(node ast.Node, comment string) {
   152  	scope := NewScope(check.scope, node.Pos(), node.End(), comment)
   153  	check.recordScope(node, scope)
   154  	check.scope = scope
   155  }
   156  
   157  func (check *Checker) closeScope() {
   158  	check.scope = check.scope.Parent()
   159  }
   160  
   161  func assignOp(op token.Token) token.Token {
   162  	// token_test.go verifies the token ordering this function relies on
   163  	if token.ADD_ASSIGN <= op && op <= token.AND_NOT_ASSIGN {
   164  		return op + (token.ADD - token.ADD_ASSIGN)
   165  	}
   166  	return token.ILLEGAL
   167  }
   168  
   169  func (check *Checker) suspendedCall(keyword string, call *ast.CallExpr) {
   170  	var x operand
   171  	var msg string
   172  	var code Code
   173  	switch check.rawExpr(nil, &x, call, nil, false) {
   174  	case conversion:
   175  		msg = "requires function call, not conversion"
   176  		code = InvalidDefer
   177  		if keyword == "go" {
   178  			code = InvalidGo
   179  		}
   180  	case expression:
   181  		msg = "discards result of"
   182  		code = UnusedResults
   183  	case statement:
   184  		return
   185  	default:
   186  		unreachable()
   187  	}
   188  	check.errorf(&x, code, "%s %s %s", keyword, msg, &x)
   189  }
   190  
   191  // goVal returns the Go value for val, or nil.
   192  func goVal(val constant.Value) any {
   193  	// val should exist, but be conservative and check
   194  	if val == nil {
   195  		return nil
   196  	}
   197  	// Match implementation restriction of other compilers.
   198  	// gc only checks duplicates for integer, floating-point
   199  	// and string values, so only create Go values for these
   200  	// types.
   201  	switch val.Kind() {
   202  	case constant.Int:
   203  		if x, ok := constant.Int64Val(val); ok {
   204  			return x
   205  		}
   206  		if x, ok := constant.Uint64Val(val); ok {
   207  			return x
   208  		}
   209  	case constant.Float:
   210  		if x, ok := constant.Float64Val(val); ok {
   211  			return x
   212  		}
   213  	case constant.String:
   214  		return constant.StringVal(val)
   215  	}
   216  	return nil
   217  }
   218  
   219  // A valueMap maps a case value (of a basic Go type) to a list of positions
   220  // where the same case value appeared, together with the corresponding case
   221  // types.
   222  // Since two case values may have the same "underlying" value but different
   223  // types we need to also check the value's types (e.g., byte(1) vs myByte(1))
   224  // when the switch expression is of interface type.
   225  type (
   226  	valueMap  map[any][]valueType // underlying Go value -> valueType
   227  	valueType struct {
   228  		pos token.Pos
   229  		typ Type
   230  	}
   231  )
   232  
   233  func (check *Checker) caseValues(x *operand, values []ast.Expr, seen valueMap) {
   234  L:
   235  	for _, e := range values {
   236  		var v operand
   237  		check.expr(nil, &v, e)
   238  		if x.mode == invalid || v.mode == invalid {
   239  			continue L
   240  		}
   241  		check.convertUntyped(&v, x.typ)
   242  		if v.mode == invalid {
   243  			continue L
   244  		}
   245  		// Order matters: By comparing v against x, error positions are at the case values.
   246  		res := v // keep original v unchanged
   247  		check.comparison(&res, x, token.EQL, true)
   248  		if res.mode == invalid {
   249  			continue L
   250  		}
   251  		if v.mode != constant_ {
   252  			continue L // we're done
   253  		}
   254  		// look for duplicate values
   255  		if val := goVal(v.val); val != nil {
   256  			// look for duplicate types for a given value
   257  			// (quadratic algorithm, but these lists tend to be very short)
   258  			for _, vt := range seen[val] {
   259  				if Identical(v.typ, vt.typ) {
   260  					check.errorf(&v, DuplicateCase, "duplicate case %s in expression switch", &v)
   261  					check.error(atPos(vt.pos), DuplicateCase, "\tprevious case") // secondary error, \t indented
   262  					continue L
   263  				}
   264  			}
   265  			seen[val] = append(seen[val], valueType{v.Pos(), v.typ})
   266  		}
   267  	}
   268  }
   269  
   270  // isNil reports whether the expression e denotes the predeclared value nil.
   271  func (check *Checker) isNil(e ast.Expr) bool {
   272  	// The only way to express the nil value is by literally writing nil (possibly in parentheses).
   273  	if name, _ := unparen(e).(*ast.Ident); name != nil {
   274  		_, ok := check.lookup(name.Name).(*Nil)
   275  		return ok
   276  	}
   277  	return false
   278  }
   279  
   280  // If the type switch expression is invalid, x is nil.
   281  func (check *Checker) caseTypes(x *operand, types []ast.Expr, seen map[Type]ast.Expr) (T Type) {
   282  	var dummy operand
   283  L:
   284  	for _, e := range types {
   285  		// The spec allows the value nil instead of a type.
   286  		if check.isNil(e) {
   287  			T = nil
   288  			check.expr(nil, &dummy, e) // run e through expr so we get the usual Info recordings
   289  		} else {
   290  			T = check.varType(e)
   291  			if !isValid(T) {
   292  				continue L
   293  			}
   294  		}
   295  		// look for duplicate types
   296  		// (quadratic algorithm, but type switches tend to be reasonably small)
   297  		for t, other := range seen {
   298  			if T == nil && t == nil || T != nil && t != nil && Identical(T, t) {
   299  				// talk about "case" rather than "type" because of nil case
   300  				Ts := "nil"
   301  				if T != nil {
   302  					Ts = TypeString(T, check.qualifier)
   303  				}
   304  				check.errorf(e, DuplicateCase, "duplicate case %s in type switch", Ts)
   305  				check.error(other, DuplicateCase, "\tprevious case") // secondary error, \t indented
   306  				continue L
   307  			}
   308  		}
   309  		seen[T] = e
   310  		if x != nil && T != nil {
   311  			check.typeAssertion(e, x, T, true)
   312  		}
   313  	}
   314  	return
   315  }
   316  
   317  // TODO(gri) Once we are certain that typeHash is correct in all situations, use this version of caseTypes instead.
   318  // (Currently it may be possible that different types have identical names and import paths due to ImporterFrom.)
   319  //
   320  // func (check *Checker) caseTypes(x *operand, xtyp *Interface, types []ast.Expr, seen map[string]ast.Expr) (T Type) {
   321  // 	var dummy operand
   322  // L:
   323  // 	for _, e := range types {
   324  // 		// The spec allows the value nil instead of a type.
   325  // 		var hash string
   326  // 		if check.isNil(e) {
   327  // 			check.expr(nil, &dummy, e) // run e through expr so we get the usual Info recordings
   328  // 			T = nil
   329  // 			hash = "<nil>" // avoid collision with a type named nil
   330  // 		} else {
   331  // 			T = check.varType(e)
   332  // 			if !isValid(T) {
   333  // 				continue L
   334  // 			}
   335  // 			hash = typeHash(T, nil)
   336  // 		}
   337  // 		// look for duplicate types
   338  // 		if other := seen[hash]; other != nil {
   339  // 			// talk about "case" rather than "type" because of nil case
   340  // 			Ts := "nil"
   341  // 			if T != nil {
   342  // 				Ts = TypeString(T, check.qualifier)
   343  // 			}
   344  // 			var err error_
   345  //			err.code = DuplicateCase
   346  // 			err.errorf(e, "duplicate case %s in type switch", Ts)
   347  // 			err.errorf(other, "previous case")
   348  // 			check.report(&err)
   349  // 			continue L
   350  // 		}
   351  // 		seen[hash] = e
   352  // 		if T != nil {
   353  // 			check.typeAssertion(e.Pos(), x, xtyp, T)
   354  // 		}
   355  // 	}
   356  // 	return
   357  // }
   358  
   359  // stmt typechecks statement s.
   360  func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
   361  	// statements must end with the same top scope as they started with
   362  	if debug {
   363  		defer func(scope *Scope) {
   364  			// don't check if code is panicking
   365  			if p := recover(); p != nil {
   366  				panic(p)
   367  			}
   368  			assert(scope == check.scope)
   369  		}(check.scope)
   370  	}
   371  
   372  	// process collected function literals before scope changes
   373  	defer check.processDelayed(len(check.delayed))
   374  
   375  	// reset context for statements of inner blocks
   376  	inner := ctxt &^ (fallthroughOk | finalSwitchCase | inTypeSwitch)
   377  
   378  	switch s := s.(type) {
   379  	case *ast.BadStmt, *ast.EmptyStmt:
   380  		// ignore
   381  
   382  	case *ast.DeclStmt:
   383  		check.declStmt(s.Decl)
   384  
   385  	case *ast.LabeledStmt:
   386  		check.hasLabel = true
   387  		check.stmt(ctxt, s.Stmt)
   388  
   389  	case *ast.ExprStmt:
   390  		// spec: "With the exception of specific built-in functions,
   391  		// function and method calls and receive operations can appear
   392  		// in statement context. Such statements may be parenthesized."
   393  		var x operand
   394  		kind := check.rawExpr(nil, &x, s.X, nil, false)
   395  		var msg string
   396  		var code Code
   397  		switch x.mode {
   398  		default:
   399  			if kind == statement {
   400  				return
   401  			}
   402  			msg = "is not used"
   403  			code = UnusedExpr
   404  		case builtin:
   405  			msg = "must be called"
   406  			code = UncalledBuiltin
   407  		case typexpr:
   408  			msg = "is not an expression"
   409  			code = NotAnExpr
   410  		}
   411  		check.errorf(&x, code, "%s %s", &x, msg)
   412  
   413  	case *ast.SendStmt:
   414  		var ch, val operand
   415  		check.expr(nil, &ch, s.Chan)
   416  		check.expr(nil, &val, s.Value)
   417  		if ch.mode == invalid || val.mode == invalid {
   418  			return
   419  		}
   420  		u := coreType(ch.typ)
   421  		if u == nil {
   422  			check.errorf(inNode(s, s.Arrow), InvalidSend, invalidOp+"cannot send to %s: no core type", &ch)
   423  			return
   424  		}
   425  		uch, _ := u.(*Chan)
   426  		if uch == nil {
   427  			check.errorf(inNode(s, s.Arrow), InvalidSend, invalidOp+"cannot send to non-channel %s", &ch)
   428  			return
   429  		}
   430  		if uch.dir == RecvOnly {
   431  			check.errorf(inNode(s, s.Arrow), InvalidSend, invalidOp+"cannot send to receive-only channel %s", &ch)
   432  			return
   433  		}
   434  		check.assignment(&val, uch.elem, "send")
   435  
   436  	case *ast.IncDecStmt:
   437  		var op token.Token
   438  		switch s.Tok {
   439  		case token.INC:
   440  			op = token.ADD
   441  		case token.DEC:
   442  			op = token.SUB
   443  		default:
   444  			check.errorf(inNode(s, s.TokPos), InvalidSyntaxTree, "unknown inc/dec operation %s", s.Tok)
   445  			return
   446  		}
   447  
   448  		var x operand
   449  		check.expr(nil, &x, s.X)
   450  		if x.mode == invalid {
   451  			return
   452  		}
   453  		if !allNumeric(x.typ) {
   454  			check.errorf(s.X, NonNumericIncDec, invalidOp+"%s%s (non-numeric type %s)", s.X, s.Tok, x.typ)
   455  			return
   456  		}
   457  
   458  		Y := &ast.BasicLit{ValuePos: s.X.Pos(), Kind: token.INT, Value: "1"} // use x's position
   459  		check.binary(&x, nil, s.X, Y, op, s.TokPos)
   460  		if x.mode == invalid {
   461  			return
   462  		}
   463  		check.assignVar(s.X, nil, &x, "assignment")
   464  
   465  	case *ast.AssignStmt:
   466  		switch s.Tok {
   467  		case token.ASSIGN, token.DEFINE:
   468  			if len(s.Lhs) == 0 {
   469  				check.error(s, InvalidSyntaxTree, "missing lhs in assignment")
   470  				return
   471  			}
   472  			if s.Tok == token.DEFINE {
   473  				check.shortVarDecl(inNode(s, s.TokPos), s.Lhs, s.Rhs)
   474  			} else {
   475  				// regular assignment
   476  				check.assignVars(s.Lhs, s.Rhs)
   477  			}
   478  
   479  		default:
   480  			// assignment operations
   481  			if len(s.Lhs) != 1 || len(s.Rhs) != 1 {
   482  				check.errorf(inNode(s, s.TokPos), MultiValAssignOp, "assignment operation %s requires single-valued expressions", s.Tok)
   483  				return
   484  			}
   485  			op := assignOp(s.Tok)
   486  			if op == token.ILLEGAL {
   487  				check.errorf(atPos(s.TokPos), InvalidSyntaxTree, "unknown assignment operation %s", s.Tok)
   488  				return
   489  			}
   490  			var x operand
   491  			check.binary(&x, nil, s.Lhs[0], s.Rhs[0], op, s.TokPos)
   492  			if x.mode == invalid {
   493  				return
   494  			}
   495  			check.assignVar(s.Lhs[0], nil, &x, "assignment")
   496  		}
   497  
   498  	case *ast.GoStmt:
   499  		check.suspendedCall("go", s.Call)
   500  
   501  	case *ast.DeferStmt:
   502  		check.suspendedCall("defer", s.Call)
   503  
   504  	case *ast.ReturnStmt:
   505  		res := check.sig.results
   506  		// Return with implicit results allowed for function with named results.
   507  		// (If one is named, all are named.)
   508  		if len(s.Results) == 0 && res.Len() > 0 && res.vars[0].name != "" {
   509  			// spec: "Implementation restriction: A compiler may disallow an empty expression
   510  			// list in a "return" statement if a different entity (constant, type, or variable)
   511  			// with the same name as a result parameter is in scope at the place of the return."
   512  			for _, obj := range res.vars {
   513  				if alt := check.lookup(obj.name); alt != nil && alt != obj {
   514  					check.errorf(s, OutOfScopeResult, "result parameter %s not in scope at return", obj.name)
   515  					check.errorf(alt, OutOfScopeResult, "\tinner declaration of %s", obj)
   516  					// ok to continue
   517  				}
   518  			}
   519  		} else {
   520  			var lhs []*Var
   521  			if res.Len() > 0 {
   522  				lhs = res.vars
   523  			}
   524  			check.initVars(lhs, s.Results, s)
   525  		}
   526  
   527  	case *ast.BranchStmt:
   528  		if s.Label != nil {
   529  			check.hasLabel = true
   530  			return // checked in 2nd pass (check.labels)
   531  		}
   532  		switch s.Tok {
   533  		case token.BREAK:
   534  			if ctxt&breakOk == 0 {
   535  				check.error(s, MisplacedBreak, "break not in for, switch, or select statement")
   536  			}
   537  		case token.CONTINUE:
   538  			if ctxt&continueOk == 0 {
   539  				check.error(s, MisplacedContinue, "continue not in for statement")
   540  			}
   541  		case token.FALLTHROUGH:
   542  			if ctxt&fallthroughOk == 0 {
   543  				var msg string
   544  				switch {
   545  				case ctxt&finalSwitchCase != 0:
   546  					msg = "cannot fallthrough final case in switch"
   547  				case ctxt&inTypeSwitch != 0:
   548  					msg = "cannot fallthrough in type switch"
   549  				default:
   550  					msg = "fallthrough statement out of place"
   551  				}
   552  				check.error(s, MisplacedFallthrough, msg)
   553  			}
   554  		default:
   555  			check.errorf(s, InvalidSyntaxTree, "branch statement: %s", s.Tok)
   556  		}
   557  
   558  	case *ast.BlockStmt:
   559  		check.openScope(s, "block")
   560  		defer check.closeScope()
   561  
   562  		check.stmtList(inner, s.List)
   563  
   564  	case *ast.IfStmt:
   565  		check.openScope(s, "if")
   566  		defer check.closeScope()
   567  
   568  		check.simpleStmt(s.Init)
   569  		var x operand
   570  		check.expr(nil, &x, s.Cond)
   571  		if x.mode != invalid && !allBoolean(x.typ) {
   572  			check.error(s.Cond, InvalidCond, "non-boolean condition in if statement")
   573  		}
   574  		check.stmt(inner, s.Body)
   575  		// The parser produces a correct AST but if it was modified
   576  		// elsewhere the else branch may be invalid. Check again.
   577  		switch s.Else.(type) {
   578  		case nil, *ast.BadStmt:
   579  			// valid or error already reported
   580  		case *ast.IfStmt, *ast.BlockStmt:
   581  			check.stmt(inner, s.Else)
   582  		default:
   583  			check.error(s.Else, InvalidSyntaxTree, "invalid else branch in if statement")
   584  		}
   585  
   586  	case *ast.SwitchStmt:
   587  		inner |= breakOk
   588  		check.openScope(s, "switch")
   589  		defer check.closeScope()
   590  
   591  		check.simpleStmt(s.Init)
   592  		var x operand
   593  		if s.Tag != nil {
   594  			check.expr(nil, &x, s.Tag)
   595  			// By checking assignment of x to an invisible temporary
   596  			// (as a compiler would), we get all the relevant checks.
   597  			check.assignment(&x, nil, "switch expression")
   598  			if x.mode != invalid && !Comparable(x.typ) && !hasNil(x.typ) {
   599  				check.errorf(&x, InvalidExprSwitch, "cannot switch on %s (%s is not comparable)", &x, x.typ)
   600  				x.mode = invalid
   601  			}
   602  		} else {
   603  			// spec: "A missing switch expression is
   604  			// equivalent to the boolean value true."
   605  			x.mode = constant_
   606  			x.typ = Typ[Bool]
   607  			x.val = constant.MakeBool(true)
   608  			x.expr = &ast.Ident{NamePos: s.Body.Lbrace, Name: "true"}
   609  		}
   610  
   611  		check.multipleDefaults(s.Body.List)
   612  
   613  		seen := make(valueMap) // map of seen case values to positions and types
   614  		for i, c := range s.Body.List {
   615  			clause, _ := c.(*ast.CaseClause)
   616  			if clause == nil {
   617  				check.error(c, InvalidSyntaxTree, "incorrect expression switch case")
   618  				continue
   619  			}
   620  			check.caseValues(&x, clause.List, seen)
   621  			check.openScope(clause, "case")
   622  			inner := inner
   623  			if i+1 < len(s.Body.List) {
   624  				inner |= fallthroughOk
   625  			} else {
   626  				inner |= finalSwitchCase
   627  			}
   628  			check.stmtList(inner, clause.Body)
   629  			check.closeScope()
   630  		}
   631  
   632  	case *ast.TypeSwitchStmt:
   633  		inner |= breakOk | inTypeSwitch
   634  		check.openScope(s, "type switch")
   635  		defer check.closeScope()
   636  
   637  		check.simpleStmt(s.Init)
   638  
   639  		// A type switch guard must be of the form:
   640  		//
   641  		//     TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
   642  		//
   643  		// The parser is checking syntactic correctness;
   644  		// remaining syntactic errors are considered AST errors here.
   645  		// TODO(gri) better factoring of error handling (invalid ASTs)
   646  		//
   647  		var lhs *ast.Ident // lhs identifier or nil
   648  		var rhs ast.Expr
   649  		switch guard := s.Assign.(type) {
   650  		case *ast.ExprStmt:
   651  			rhs = guard.X
   652  		case *ast.AssignStmt:
   653  			if len(guard.Lhs) != 1 || guard.Tok != token.DEFINE || len(guard.Rhs) != 1 {
   654  				check.error(s, InvalidSyntaxTree, "incorrect form of type switch guard")
   655  				return
   656  			}
   657  
   658  			lhs, _ = guard.Lhs[0].(*ast.Ident)
   659  			if lhs == nil {
   660  				check.error(s, InvalidSyntaxTree, "incorrect form of type switch guard")
   661  				return
   662  			}
   663  
   664  			if lhs.Name == "_" {
   665  				// _ := x.(type) is an invalid short variable declaration
   666  				check.softErrorf(lhs, NoNewVar, "no new variable on left side of :=")
   667  				lhs = nil // avoid declared and not used error below
   668  			} else {
   669  				check.recordDef(lhs, nil) // lhs variable is implicitly declared in each cause clause
   670  			}
   671  
   672  			rhs = guard.Rhs[0]
   673  
   674  		default:
   675  			check.error(s, InvalidSyntaxTree, "incorrect form of type switch guard")
   676  			return
   677  		}
   678  
   679  		// rhs must be of the form: expr.(type) and expr must be an ordinary interface
   680  		expr, _ := rhs.(*ast.TypeAssertExpr)
   681  		if expr == nil || expr.Type != nil {
   682  			check.error(s, InvalidSyntaxTree, "incorrect form of type switch guard")
   683  			return
   684  		}
   685  		var x operand
   686  		check.expr(nil, &x, expr.X)
   687  		if x.mode == invalid {
   688  			return
   689  		}
   690  		// TODO(gri) we may want to permit type switches on type parameter values at some point
   691  		var sx *operand // switch expression against which cases are compared against; nil if invalid
   692  		if isTypeParam(x.typ) {
   693  			check.errorf(&x, InvalidTypeSwitch, "cannot use type switch on type parameter value %s", &x)
   694  		} else {
   695  			if _, ok := under(x.typ).(*Interface); ok {
   696  				sx = &x
   697  			} else {
   698  				check.errorf(&x, InvalidTypeSwitch, "%s is not an interface", &x)
   699  			}
   700  		}
   701  
   702  		check.multipleDefaults(s.Body.List)
   703  
   704  		var lhsVars []*Var              // list of implicitly declared lhs variables
   705  		seen := make(map[Type]ast.Expr) // map of seen types to positions
   706  		for _, s := range s.Body.List {
   707  			clause, _ := s.(*ast.CaseClause)
   708  			if clause == nil {
   709  				check.error(s, InvalidSyntaxTree, "incorrect type switch case")
   710  				continue
   711  			}
   712  			// Check each type in this type switch case.
   713  			T := check.caseTypes(sx, clause.List, seen)
   714  			check.openScope(clause, "case")
   715  			// If lhs exists, declare a corresponding variable in the case-local scope.
   716  			if lhs != nil {
   717  				// spec: "The TypeSwitchGuard may include a short variable declaration.
   718  				// When that form is used, the variable is declared at the beginning of
   719  				// the implicit block in each clause. In clauses with a case listing
   720  				// exactly one type, the variable has that type; otherwise, the variable
   721  				// has the type of the expression in the TypeSwitchGuard."
   722  				if len(clause.List) != 1 || T == nil {
   723  					T = x.typ
   724  				}
   725  				obj := NewVar(lhs.Pos(), check.pkg, lhs.Name, T)
   726  				scopePos := clause.Pos() + token.Pos(len("default")) // for default clause (len(List) == 0)
   727  				if n := len(clause.List); n > 0 {
   728  					scopePos = clause.List[n-1].End()
   729  				}
   730  				check.declare(check.scope, nil, obj, scopePos)
   731  				check.recordImplicit(clause, obj)
   732  				// For the "declared and not used" error, all lhs variables act as
   733  				// one; i.e., if any one of them is 'used', all of them are 'used'.
   734  				// Collect them for later analysis.
   735  				lhsVars = append(lhsVars, obj)
   736  			}
   737  			check.stmtList(inner, clause.Body)
   738  			check.closeScope()
   739  		}
   740  
   741  		// If lhs exists, we must have at least one lhs variable that was used.
   742  		if lhs != nil {
   743  			var used bool
   744  			for _, v := range lhsVars {
   745  				if v.used {
   746  					used = true
   747  				}
   748  				v.used = true // avoid usage error when checking entire function
   749  			}
   750  			if !used {
   751  				check.softErrorf(lhs, UnusedVar, "%s declared and not used", lhs.Name)
   752  			}
   753  		}
   754  
   755  	case *ast.SelectStmt:
   756  		inner |= breakOk
   757  
   758  		check.multipleDefaults(s.Body.List)
   759  
   760  		for _, s := range s.Body.List {
   761  			clause, _ := s.(*ast.CommClause)
   762  			if clause == nil {
   763  				continue // error reported before
   764  			}
   765  
   766  			// clause.Comm must be a SendStmt, RecvStmt, or default case
   767  			valid := false
   768  			var rhs ast.Expr // rhs of RecvStmt, or nil
   769  			switch s := clause.Comm.(type) {
   770  			case nil, *ast.SendStmt:
   771  				valid = true
   772  			case *ast.AssignStmt:
   773  				if len(s.Rhs) == 1 {
   774  					rhs = s.Rhs[0]
   775  				}
   776  			case *ast.ExprStmt:
   777  				rhs = s.X
   778  			}
   779  
   780  			// if present, rhs must be a receive operation
   781  			if rhs != nil {
   782  				if x, _ := unparen(rhs).(*ast.UnaryExpr); x != nil && x.Op == token.ARROW {
   783  					valid = true
   784  				}
   785  			}
   786  
   787  			if !valid {
   788  				check.error(clause.Comm, InvalidSelectCase, "select case must be send or receive (possibly with assignment)")
   789  				continue
   790  			}
   791  
   792  			check.openScope(s, "case")
   793  			if clause.Comm != nil {
   794  				check.stmt(inner, clause.Comm)
   795  			}
   796  			check.stmtList(inner, clause.Body)
   797  			check.closeScope()
   798  		}
   799  
   800  	case *ast.ForStmt:
   801  		inner |= breakOk | continueOk
   802  		check.openScope(s, "for")
   803  		defer check.closeScope()
   804  
   805  		check.simpleStmt(s.Init)
   806  		if s.Cond != nil {
   807  			var x operand
   808  			check.expr(nil, &x, s.Cond)
   809  			if x.mode != invalid && !allBoolean(x.typ) {
   810  				check.error(s.Cond, InvalidCond, "non-boolean condition in for statement")
   811  			}
   812  		}
   813  		check.simpleStmt(s.Post)
   814  		// spec: "The init statement may be a short variable
   815  		// declaration, but the post statement must not."
   816  		if s, _ := s.Post.(*ast.AssignStmt); s != nil && s.Tok == token.DEFINE {
   817  			check.softErrorf(s, InvalidPostDecl, "cannot declare in post statement")
   818  			// Don't call useLHS here because we want to use the lhs in
   819  			// this erroneous statement so that we don't get errors about
   820  			// these lhs variables being declared and not used.
   821  			check.use(s.Lhs...) // avoid follow-up errors
   822  		}
   823  		check.stmt(inner, s.Body)
   824  
   825  	case *ast.RangeStmt:
   826  		inner |= breakOk | continueOk
   827  		check.rangeStmt(inner, s)
   828  
   829  	default:
   830  		check.error(s, InvalidSyntaxTree, "invalid statement")
   831  	}
   832  }
   833  
   834  func (check *Checker) rangeStmt(inner stmtContext, s *ast.RangeStmt) {
   835  	// Convert go/ast form to local variables.
   836  	type Expr = ast.Expr
   837  	type identType = ast.Ident
   838  	identName := func(n *identType) string { return n.Name }
   839  	sKey, sValue := s.Key, s.Value
   840  	var sExtra ast.Expr = nil
   841  	isDef := s.Tok == token.DEFINE
   842  	rangeVar := s.X
   843  	noNewVarPos := inNode(s, s.TokPos)
   844  
   845  	// Everything from here on is shared between cmd/compile/internal/types2 and go/types.
   846  
   847  	// check expression to iterate over
   848  	var x operand
   849  	check.expr(nil, &x, rangeVar)
   850  
   851  	// determine key/value types
   852  	var key, val Type
   853  	if x.mode != invalid {
   854  		// Ranging over a type parameter is permitted if it has a core type.
   855  		k, v, cause, isFunc, ok := rangeKeyVal(x.typ, func(v goVersion) bool {
   856  			return check.allowVersion(check.pkg, x.expr, v)
   857  		})
   858  		switch {
   859  		case !ok && cause != "":
   860  			check.softErrorf(&x, InvalidRangeExpr, "cannot range over %s: %s", &x, cause)
   861  		case !ok:
   862  			check.softErrorf(&x, InvalidRangeExpr, "cannot range over %s", &x)
   863  		case k == nil && sKey != nil:
   864  			check.softErrorf(sKey, InvalidIterVar, "range over %s permits no iteration variables", &x)
   865  		case v == nil && sValue != nil:
   866  			check.softErrorf(sValue, InvalidIterVar, "range over %s permits only one iteration variable", &x)
   867  		case sExtra != nil:
   868  			check.softErrorf(sExtra, InvalidIterVar, "range clause permits at most two iteration variables")
   869  		case isFunc && ((k == nil) != (sKey == nil) || (v == nil) != (sValue == nil)):
   870  			var count string
   871  			switch {
   872  			case k == nil:
   873  				count = "no iteration variables"
   874  			case v == nil:
   875  				count = "one iteration variable"
   876  			default:
   877  				count = "two iteration variables"
   878  			}
   879  			check.softErrorf(&x, InvalidIterVar, "range over %s must have %s", &x, count)
   880  		}
   881  		key, val = k, v
   882  	}
   883  
   884  	// Open the for-statement block scope now, after the range clause.
   885  	// Iteration variables declared with := need to go in this scope (was go.dev/issue/51437).
   886  	check.openScope(s, "range")
   887  	defer check.closeScope()
   888  
   889  	// check assignment to/declaration of iteration variables
   890  	// (irregular assignment, cannot easily map to existing assignment checks)
   891  
   892  	// lhs expressions and initialization value (rhs) types
   893  	lhs := [2]Expr{sKey, sValue} // sKey, sValue may be nil
   894  	rhs := [2]Type{key, val}     // key, val may be nil
   895  
   896  	constIntRange := x.mode == constant_ && isInteger(x.typ)
   897  
   898  	if isDef {
   899  		// short variable declaration
   900  		var vars []*Var
   901  		for i, lhs := range lhs {
   902  			if lhs == nil {
   903  				continue
   904  			}
   905  
   906  			// determine lhs variable
   907  			var obj *Var
   908  			if ident, _ := lhs.(*identType); ident != nil {
   909  				// declare new variable
   910  				name := identName(ident)
   911  				obj = NewVar(ident.Pos(), check.pkg, name, nil)
   912  				check.recordDef(ident, obj)
   913  				// _ variables don't count as new variables
   914  				if name != "_" {
   915  					vars = append(vars, obj)
   916  				}
   917  			} else {
   918  				check.errorf(lhs, InvalidSyntaxTree, "cannot declare %s", lhs)
   919  				obj = NewVar(lhs.Pos(), check.pkg, "_", nil) // dummy variable
   920  			}
   921  
   922  			// initialize lhs variable
   923  			if constIntRange {
   924  				check.initVar(obj, &x, "range clause")
   925  			} else if typ := rhs[i]; typ != nil {
   926  				x.mode = value
   927  				x.expr = lhs // we don't have a better rhs expression to use here
   928  				x.typ = typ
   929  				check.initVar(obj, &x, "assignment") // error is on variable, use "assignment" not "range clause"
   930  			} else {
   931  				obj.typ = Typ[Invalid]
   932  				obj.used = true // don't complain about unused variable
   933  			}
   934  		}
   935  
   936  		// declare variables
   937  		if len(vars) > 0 {
   938  			scopePos := s.Body.Pos()
   939  			for _, obj := range vars {
   940  				check.declare(check.scope, nil /* recordDef already called */, obj, scopePos)
   941  			}
   942  		} else {
   943  			check.error(noNewVarPos, NoNewVar, "no new variables on left side of :=")
   944  		}
   945  	} else if sKey != nil /* lhs[0] != nil */ {
   946  		// ordinary assignment
   947  		for i, lhs := range lhs {
   948  			if lhs == nil {
   949  				continue
   950  			}
   951  
   952  			if constIntRange {
   953  				check.assignVar(lhs, nil, &x, "range clause")
   954  			} else if typ := rhs[i]; typ != nil {
   955  				x.mode = value
   956  				x.expr = lhs // we don't have a better rhs expression to use here
   957  				x.typ = typ
   958  				check.assignVar(lhs, nil, &x, "assignment") // error is on variable, use "assignment" not "range clause"
   959  			}
   960  		}
   961  	} else if constIntRange {
   962  		// If we don't have any iteration variables, we still need to
   963  		// check that a (possibly untyped) integer range expression x
   964  		// is valid.
   965  		// We do this by checking the assignment _ = x. This ensures
   966  		// that an untyped x can be converted to a value of type int.
   967  		check.assignment(&x, nil, "range clause")
   968  	}
   969  
   970  	check.stmt(inner, s.Body)
   971  }
   972  
   973  // rangeKeyVal returns the key and value type produced by a range clause
   974  // over an expression of type typ.
   975  // If allowVersion != nil, it is used to check the required language version.
   976  // If the range clause is not permitted, rangeKeyVal returns ok = false.
   977  // When ok = false, rangeKeyVal may also return a reason in cause.
   978  func rangeKeyVal(typ Type, allowVersion func(goVersion) bool) (key, val Type, cause string, isFunc, ok bool) {
   979  	bad := func(cause string) (Type, Type, string, bool, bool) {
   980  		return Typ[Invalid], Typ[Invalid], cause, false, false
   981  	}
   982  	toSig := func(t Type) *Signature {
   983  		sig, _ := coreType(t).(*Signature)
   984  		return sig
   985  	}
   986  
   987  	orig := typ
   988  	switch typ := arrayPtrDeref(coreType(typ)).(type) {
   989  	case nil:
   990  		return bad("no core type")
   991  	case *Basic:
   992  		if isString(typ) {
   993  			return Typ[Int], universeRune, "", false, true // use 'rune' name
   994  		}
   995  		if isInteger(typ) {
   996  			if allowVersion != nil && !allowVersion(go1_22) {
   997  				return bad("requires go1.22 or later")
   998  			}
   999  			return orig, nil, "", false, true
  1000  		}
  1001  	case *Array:
  1002  		return Typ[Int], typ.elem, "", false, true
  1003  	case *Slice:
  1004  		return Typ[Int], typ.elem, "", false, true
  1005  	case *Map:
  1006  		return typ.key, typ.elem, "", false, true
  1007  	case *Chan:
  1008  		if typ.dir == SendOnly {
  1009  			return bad("receive from send-only channel")
  1010  		}
  1011  		return typ.elem, nil, "", false, true
  1012  	case *Signature:
  1013  		// TODO(gri) when this becomes enabled permanently, add version check
  1014  		if !buildcfg.Experiment.RangeFunc {
  1015  			break
  1016  		}
  1017  		assert(typ.Recv() == nil)
  1018  		switch {
  1019  		case typ.Params().Len() != 1:
  1020  			return bad("func must be func(yield func(...) bool): wrong argument count")
  1021  		case toSig(typ.Params().At(0).Type()) == nil:
  1022  			return bad("func must be func(yield func(...) bool): argument is not func")
  1023  		case typ.Results().Len() != 0:
  1024  			return bad("func must be func(yield func(...) bool): unexpected results")
  1025  		}
  1026  		cb := toSig(typ.Params().At(0).Type())
  1027  		assert(cb.Recv() == nil)
  1028  		switch {
  1029  		case cb.Params().Len() > 2:
  1030  			return bad("func must be func(yield func(...) bool): yield func has too many parameters")
  1031  		case cb.Results().Len() != 1 || !isBoolean(cb.Results().At(0).Type()):
  1032  			return bad("func must be func(yield func(...) bool): yield func does not return bool")
  1033  		}
  1034  		if cb.Params().Len() >= 1 {
  1035  			key = cb.Params().At(0).Type()
  1036  		}
  1037  		if cb.Params().Len() >= 2 {
  1038  			val = cb.Params().At(1).Type()
  1039  		}
  1040  		return key, val, "", true, true
  1041  	}
  1042  	return
  1043  }
  1044  

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