Source file src/go/types/decl.go

Documentation: go/types

     1  // Copyright 2014 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 types
     6  
     7  import (
     8  	"fmt"
     9  	"go/ast"
    10  	"go/constant"
    11  	"go/token"
    12  	. "internal/types/errors"
    13  )
    14  
    15  func (check *Checker) reportAltDecl(obj Object) {
    16  	if pos := obj.Pos(); pos.IsValid() {
    17  		// We use "other" rather than "previous" here because
    18  		// the first declaration seen may not be textually
    19  		// earlier in the source.
    20  		check.errorf(obj, DuplicateDecl, "\tother declaration of %s", obj.Name()) // secondary error, \t indented
    21  	}
    22  }
    23  
    24  func (check *Checker) declare(scope *Scope, id *ast.Ident, obj Object, pos token.Pos) {
    25  	// spec: "The blank identifier, represented by the underscore
    26  	// character _, may be used in a declaration like any other
    27  	// identifier but the declaration does not introduce a new
    28  	// binding."
    29  	if obj.Name() != "_" {
    30  		if alt := scope.Insert(obj); alt != nil {
    31  			check.errorf(obj, DuplicateDecl, "%s redeclared in this block", obj.Name())
    32  			check.reportAltDecl(alt)
    33  			return
    34  		}
    35  		obj.setScopePos(pos)
    36  	}
    37  	if id != nil {
    38  		check.recordDef(id, obj)
    39  	}
    40  }
    41  
    42  // pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
    43  func pathString(path []Object) string {
    44  	var s string
    45  	for i, p := range path {
    46  		if i > 0 {
    47  			s += "->"
    48  		}
    49  		s += p.Name()
    50  	}
    51  	return s
    52  }
    53  
    54  // objDecl type-checks the declaration of obj in its respective (file) environment.
    55  // For the meaning of def, see Checker.definedType, in typexpr.go.
    56  func (check *Checker) objDecl(obj Object, def *TypeName) {
    57  	if check.conf._Trace && obj.Type() == nil {
    58  		if check.indent == 0 {
    59  			fmt.Println() // empty line between top-level objects for readability
    60  		}
    61  		check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
    62  		check.indent++
    63  		defer func() {
    64  			check.indent--
    65  			check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
    66  		}()
    67  	}
    68  
    69  	// Checking the declaration of obj means inferring its type
    70  	// (and possibly its value, for constants).
    71  	// An object's type (and thus the object) may be in one of
    72  	// three states which are expressed by colors:
    73  	//
    74  	// - an object whose type is not yet known is painted white (initial color)
    75  	// - an object whose type is in the process of being inferred is painted grey
    76  	// - an object whose type is fully inferred is painted black
    77  	//
    78  	// During type inference, an object's color changes from white to grey
    79  	// to black (pre-declared objects are painted black from the start).
    80  	// A black object (i.e., its type) can only depend on (refer to) other black
    81  	// ones. White and grey objects may depend on white and black objects.
    82  	// A dependency on a grey object indicates a cycle which may or may not be
    83  	// valid.
    84  	//
    85  	// When objects turn grey, they are pushed on the object path (a stack);
    86  	// they are popped again when they turn black. Thus, if a grey object (a
    87  	// cycle) is encountered, it is on the object path, and all the objects
    88  	// it depends on are the remaining objects on that path. Color encoding
    89  	// is such that the color value of a grey object indicates the index of
    90  	// that object in the object path.
    91  
    92  	// During type-checking, white objects may be assigned a type without
    93  	// traversing through objDecl; e.g., when initializing constants and
    94  	// variables. Update the colors of those objects here (rather than
    95  	// everywhere where we set the type) to satisfy the color invariants.
    96  	if obj.color() == white && obj.Type() != nil {
    97  		obj.setColor(black)
    98  		return
    99  	}
   100  
   101  	switch obj.color() {
   102  	case white:
   103  		assert(obj.Type() == nil)
   104  		// All color values other than white and black are considered grey.
   105  		// Because black and white are < grey, all values >= grey are grey.
   106  		// Use those values to encode the object's index into the object path.
   107  		obj.setColor(grey + color(check.push(obj)))
   108  		defer func() {
   109  			check.pop().setColor(black)
   110  		}()
   111  
   112  	case black:
   113  		assert(obj.Type() != nil)
   114  		return
   115  
   116  	default:
   117  		// Color values other than white or black are considered grey.
   118  		fallthrough
   119  
   120  	case grey:
   121  		// We have a (possibly invalid) cycle.
   122  		// In the existing code, this is marked by a non-nil type
   123  		// for the object except for constants and variables whose
   124  		// type may be non-nil (known), or nil if it depends on the
   125  		// not-yet known initialization value.
   126  		// In the former case, set the type to Typ[Invalid] because
   127  		// we have an initialization cycle. The cycle error will be
   128  		// reported later, when determining initialization order.
   129  		// TODO(gri) Report cycle here and simplify initialization
   130  		// order code.
   131  		switch obj := obj.(type) {
   132  		case *Const:
   133  			if !check.validCycle(obj) || obj.typ == nil {
   134  				obj.typ = Typ[Invalid]
   135  			}
   136  
   137  		case *Var:
   138  			if !check.validCycle(obj) || obj.typ == nil {
   139  				obj.typ = Typ[Invalid]
   140  			}
   141  
   142  		case *TypeName:
   143  			if !check.validCycle(obj) {
   144  				// break cycle
   145  				// (without this, calling underlying()
   146  				// below may lead to an endless loop
   147  				// if we have a cycle for a defined
   148  				// (*Named) type)
   149  				obj.typ = Typ[Invalid]
   150  			}
   151  
   152  		case *Func:
   153  			if !check.validCycle(obj) {
   154  				// Don't set obj.typ to Typ[Invalid] here
   155  				// because plenty of code type-asserts that
   156  				// functions have a *Signature type. Grey
   157  				// functions have their type set to an empty
   158  				// signature which makes it impossible to
   159  				// initialize a variable with the function.
   160  			}
   161  
   162  		default:
   163  			unreachable()
   164  		}
   165  		assert(obj.Type() != nil)
   166  		return
   167  	}
   168  
   169  	d := check.objMap[obj]
   170  	if d == nil {
   171  		check.dump("%v: %s should have been declared", obj.Pos(), obj)
   172  		unreachable()
   173  	}
   174  
   175  	// save/restore current environment and set up object environment
   176  	defer func(env environment) {
   177  		check.environment = env
   178  	}(check.environment)
   179  	check.environment = environment{
   180  		scope: d.file,
   181  	}
   182  
   183  	// Const and var declarations must not have initialization
   184  	// cycles. We track them by remembering the current declaration
   185  	// in check.decl. Initialization expressions depending on other
   186  	// consts, vars, or functions, add dependencies to the current
   187  	// check.decl.
   188  	switch obj := obj.(type) {
   189  	case *Const:
   190  		check.decl = d // new package-level const decl
   191  		check.constDecl(obj, d.vtyp, d.init, d.inherited)
   192  	case *Var:
   193  		check.decl = d // new package-level var decl
   194  		check.varDecl(obj, d.lhs, d.vtyp, d.init)
   195  	case *TypeName:
   196  		// invalid recursive types are detected via path
   197  		check.typeDecl(obj, d.tdecl, def)
   198  		check.collectMethods(obj) // methods can only be added to top-level types
   199  	case *Func:
   200  		// functions may be recursive - no need to track dependencies
   201  		check.funcDecl(obj, d)
   202  	default:
   203  		unreachable()
   204  	}
   205  }
   206  
   207  // validCycle checks if the cycle starting with obj is valid and
   208  // reports an error if it is not.
   209  func (check *Checker) validCycle(obj Object) (valid bool) {
   210  	// The object map contains the package scope objects and the non-interface methods.
   211  	if debug {
   212  		info := check.objMap[obj]
   213  		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
   214  		isPkgObj := obj.Parent() == check.pkg.scope
   215  		if isPkgObj != inObjMap {
   216  			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
   217  			unreachable()
   218  		}
   219  	}
   220  
   221  	// Count cycle objects.
   222  	assert(obj.color() >= grey)
   223  	start := obj.color() - grey // index of obj in objPath
   224  	cycle := check.objPath[start:]
   225  	tparCycle := false // if set, the cycle is through a type parameter list
   226  	nval := 0          // number of (constant or variable) values in the cycle; valid if !generic
   227  	ndef := 0          // number of type definitions in the cycle; valid if !generic
   228  loop:
   229  	for _, obj := range cycle {
   230  		switch obj := obj.(type) {
   231  		case *Const, *Var:
   232  			nval++
   233  		case *TypeName:
   234  			// If we reach a generic type that is part of a cycle
   235  			// and we are in a type parameter list, we have a cycle
   236  			// through a type parameter list, which is invalid.
   237  			if check.inTParamList && isGeneric(obj.typ) {
   238  				tparCycle = true
   239  				break loop
   240  			}
   241  
   242  			// Determine if the type name is an alias or not. For
   243  			// package-level objects, use the object map which
   244  			// provides syntactic information (which doesn't rely
   245  			// on the order in which the objects are set up). For
   246  			// local objects, we can rely on the order, so use
   247  			// the object's predicate.
   248  			// TODO(gri) It would be less fragile to always access
   249  			// the syntactic information. We should consider storing
   250  			// this information explicitly in the object.
   251  			var alias bool
   252  			if check.enableAlias {
   253  				alias = obj.IsAlias()
   254  			} else {
   255  				if d := check.objMap[obj]; d != nil {
   256  					alias = d.tdecl.Assign.IsValid() // package-level object
   257  				} else {
   258  					alias = obj.IsAlias() // function local object
   259  				}
   260  			}
   261  			if !alias {
   262  				ndef++
   263  			}
   264  		case *Func:
   265  			// ignored for now
   266  		default:
   267  			unreachable()
   268  		}
   269  	}
   270  
   271  	if check.conf._Trace {
   272  		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
   273  		if tparCycle {
   274  			check.trace(obj.Pos(), "## cycle contains: generic type in a type parameter list")
   275  		} else {
   276  			check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
   277  		}
   278  		defer func() {
   279  			if valid {
   280  				check.trace(obj.Pos(), "=> cycle is valid")
   281  			} else {
   282  				check.trace(obj.Pos(), "=> error: cycle is invalid")
   283  			}
   284  		}()
   285  	}
   286  
   287  	if !tparCycle {
   288  		// A cycle involving only constants and variables is invalid but we
   289  		// ignore them here because they are reported via the initialization
   290  		// cycle check.
   291  		if nval == len(cycle) {
   292  			return true
   293  		}
   294  
   295  		// A cycle involving only types (and possibly functions) must have at least
   296  		// one type definition to be permitted: If there is no type definition, we
   297  		// have a sequence of alias type names which will expand ad infinitum.
   298  		if nval == 0 && ndef > 0 {
   299  			return true
   300  		}
   301  	}
   302  
   303  	check.cycleError(cycle)
   304  	return false
   305  }
   306  
   307  // cycleError reports a declaration cycle starting with
   308  // the object in cycle that is "first" in the source.
   309  func (check *Checker) cycleError(cycle []Object) {
   310  	// name returns the (possibly qualified) object name.
   311  	// This is needed because with generic types, cycles
   312  	// may refer to imported types. See go.dev/issue/50788.
   313  	// TODO(gri) Thus functionality is used elsewhere. Factor it out.
   314  	name := func(obj Object) string {
   315  		return packagePrefix(obj.Pkg(), check.qualifier) + obj.Name()
   316  	}
   317  
   318  	// TODO(gri) Should we start with the last (rather than the first) object in the cycle
   319  	//           since that is the earliest point in the source where we start seeing the
   320  	//           cycle? That would be more consistent with other error messages.
   321  	i := firstInSrc(cycle)
   322  	obj := cycle[i]
   323  	objName := name(obj)
   324  	// If obj is a type alias, mark it as valid (not broken) in order to avoid follow-on errors.
   325  	tname, _ := obj.(*TypeName)
   326  	if tname != nil && tname.IsAlias() {
   327  		// If we use Alias nodes, it is initialized with Typ[Invalid].
   328  		// TODO(gri) Adjust this code if we initialize with nil.
   329  		if !check.enableAlias {
   330  			check.validAlias(tname, Typ[Invalid])
   331  		}
   332  	}
   333  
   334  	// report a more concise error for self references
   335  	if len(cycle) == 1 {
   336  		if tname != nil {
   337  			check.errorf(obj, InvalidDeclCycle, "invalid recursive type: %s refers to itself", objName)
   338  		} else {
   339  			check.errorf(obj, InvalidDeclCycle, "invalid cycle in declaration: %s refers to itself", objName)
   340  		}
   341  		return
   342  	}
   343  
   344  	if tname != nil {
   345  		check.errorf(obj, InvalidDeclCycle, "invalid recursive type %s", objName)
   346  	} else {
   347  		check.errorf(obj, InvalidDeclCycle, "invalid cycle in declaration of %s", objName)
   348  	}
   349  	for range cycle {
   350  		check.errorf(obj, InvalidDeclCycle, "\t%s refers to", objName) // secondary error, \t indented
   351  		i++
   352  		if i >= len(cycle) {
   353  			i = 0
   354  		}
   355  		obj = cycle[i]
   356  		objName = name(obj)
   357  	}
   358  	check.errorf(obj, InvalidDeclCycle, "\t%s", objName)
   359  }
   360  
   361  // firstInSrc reports the index of the object with the "smallest"
   362  // source position in path. path must not be empty.
   363  func firstInSrc(path []Object) int {
   364  	fst, pos := 0, path[0].Pos()
   365  	for i, t := range path[1:] {
   366  		if cmpPos(t.Pos(), pos) < 0 {
   367  			fst, pos = i+1, t.Pos()
   368  		}
   369  	}
   370  	return fst
   371  }
   372  
   373  type (
   374  	decl interface {
   375  		node() ast.Node
   376  	}
   377  
   378  	importDecl struct{ spec *ast.ImportSpec }
   379  	constDecl  struct {
   380  		spec      *ast.ValueSpec
   381  		iota      int
   382  		typ       ast.Expr
   383  		init      []ast.Expr
   384  		inherited bool
   385  	}
   386  	varDecl  struct{ spec *ast.ValueSpec }
   387  	typeDecl struct{ spec *ast.TypeSpec }
   388  	funcDecl struct{ decl *ast.FuncDecl }
   389  )
   390  
   391  func (d importDecl) node() ast.Node { return d.spec }
   392  func (d constDecl) node() ast.Node  { return d.spec }
   393  func (d varDecl) node() ast.Node    { return d.spec }
   394  func (d typeDecl) node() ast.Node   { return d.spec }
   395  func (d funcDecl) node() ast.Node   { return d.decl }
   396  
   397  func (check *Checker) walkDecls(decls []ast.Decl, f func(decl)) {
   398  	for _, d := range decls {
   399  		check.walkDecl(d, f)
   400  	}
   401  }
   402  
   403  func (check *Checker) walkDecl(d ast.Decl, f func(decl)) {
   404  	switch d := d.(type) {
   405  	case *ast.BadDecl:
   406  		// ignore
   407  	case *ast.GenDecl:
   408  		var last *ast.ValueSpec // last ValueSpec with type or init exprs seen
   409  		for iota, s := range d.Specs {
   410  			switch s := s.(type) {
   411  			case *ast.ImportSpec:
   412  				f(importDecl{s})
   413  			case *ast.ValueSpec:
   414  				switch d.Tok {
   415  				case token.CONST:
   416  					// determine which initialization expressions to use
   417  					inherited := true
   418  					switch {
   419  					case s.Type != nil || len(s.Values) > 0:
   420  						last = s
   421  						inherited = false
   422  					case last == nil:
   423  						last = new(ast.ValueSpec) // make sure last exists
   424  						inherited = false
   425  					}
   426  					check.arityMatch(s, last)
   427  					f(constDecl{spec: s, iota: iota, typ: last.Type, init: last.Values, inherited: inherited})
   428  				case token.VAR:
   429  					check.arityMatch(s, nil)
   430  					f(varDecl{s})
   431  				default:
   432  					check.errorf(s, InvalidSyntaxTree, "invalid token %s", d.Tok)
   433  				}
   434  			case *ast.TypeSpec:
   435  				f(typeDecl{s})
   436  			default:
   437  				check.errorf(s, InvalidSyntaxTree, "unknown ast.Spec node %T", s)
   438  			}
   439  		}
   440  	case *ast.FuncDecl:
   441  		f(funcDecl{d})
   442  	default:
   443  		check.errorf(d, InvalidSyntaxTree, "unknown ast.Decl node %T", d)
   444  	}
   445  }
   446  
   447  func (check *Checker) constDecl(obj *Const, typ, init ast.Expr, inherited bool) {
   448  	assert(obj.typ == nil)
   449  
   450  	// use the correct value of iota
   451  	defer func(iota constant.Value, errpos positioner) {
   452  		check.iota = iota
   453  		check.errpos = errpos
   454  	}(check.iota, check.errpos)
   455  	check.iota = obj.val
   456  	check.errpos = nil
   457  
   458  	// provide valid constant value under all circumstances
   459  	obj.val = constant.MakeUnknown()
   460  
   461  	// determine type, if any
   462  	if typ != nil {
   463  		t := check.typ(typ)
   464  		if !isConstType(t) {
   465  			// don't report an error if the type is an invalid C (defined) type
   466  			// (go.dev/issue/22090)
   467  			if isValid(under(t)) {
   468  				check.errorf(typ, InvalidConstType, "invalid constant type %s", t)
   469  			}
   470  			obj.typ = Typ[Invalid]
   471  			return
   472  		}
   473  		obj.typ = t
   474  	}
   475  
   476  	// check initialization
   477  	var x operand
   478  	if init != nil {
   479  		if inherited {
   480  			// The initialization expression is inherited from a previous
   481  			// constant declaration, and (error) positions refer to that
   482  			// expression and not the current constant declaration. Use
   483  			// the constant identifier position for any errors during
   484  			// init expression evaluation since that is all we have
   485  			// (see issues go.dev/issue/42991, go.dev/issue/42992).
   486  			check.errpos = atPos(obj.pos)
   487  		}
   488  		check.expr(nil, &x, init)
   489  	}
   490  	check.initConst(obj, &x)
   491  }
   492  
   493  func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
   494  	assert(obj.typ == nil)
   495  
   496  	// determine type, if any
   497  	if typ != nil {
   498  		obj.typ = check.varType(typ)
   499  		// We cannot spread the type to all lhs variables if there
   500  		// are more than one since that would mark them as checked
   501  		// (see Checker.objDecl) and the assignment of init exprs,
   502  		// if any, would not be checked.
   503  		//
   504  		// TODO(gri) If we have no init expr, we should distribute
   505  		// a given type otherwise we need to re-evalate the type
   506  		// expr for each lhs variable, leading to duplicate work.
   507  	}
   508  
   509  	// check initialization
   510  	if init == nil {
   511  		if typ == nil {
   512  			// error reported before by arityMatch
   513  			obj.typ = Typ[Invalid]
   514  		}
   515  		return
   516  	}
   517  
   518  	if lhs == nil || len(lhs) == 1 {
   519  		assert(lhs == nil || lhs[0] == obj)
   520  		var x operand
   521  		check.expr(newTarget(obj.typ, obj.name), &x, init)
   522  		check.initVar(obj, &x, "variable declaration")
   523  		return
   524  	}
   525  
   526  	if debug {
   527  		// obj must be one of lhs
   528  		found := false
   529  		for _, lhs := range lhs {
   530  			if obj == lhs {
   531  				found = true
   532  				break
   533  			}
   534  		}
   535  		if !found {
   536  			panic("inconsistent lhs")
   537  		}
   538  	}
   539  
   540  	// We have multiple variables on the lhs and one init expr.
   541  	// Make sure all variables have been given the same type if
   542  	// one was specified, otherwise they assume the type of the
   543  	// init expression values (was go.dev/issue/15755).
   544  	if typ != nil {
   545  		for _, lhs := range lhs {
   546  			lhs.typ = obj.typ
   547  		}
   548  	}
   549  
   550  	check.initVars(lhs, []ast.Expr{init}, nil)
   551  }
   552  
   553  // isImportedConstraint reports whether typ is an imported type constraint.
   554  func (check *Checker) isImportedConstraint(typ Type) bool {
   555  	named := asNamed(typ)
   556  	if named == nil || named.obj.pkg == check.pkg || named.obj.pkg == nil {
   557  		return false
   558  	}
   559  	u, _ := named.under().(*Interface)
   560  	return u != nil && !u.IsMethodSet()
   561  }
   562  
   563  func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName) {
   564  	assert(obj.typ == nil)
   565  
   566  	var rhs Type
   567  	check.later(func() {
   568  		if t := asNamed(obj.typ); t != nil { // type may be invalid
   569  			check.validType(t)
   570  		}
   571  		// If typ is local, an error was already reported where typ is specified/defined.
   572  		_ = check.isImportedConstraint(rhs) && check.verifyVersionf(tdecl.Type, go1_18, "using type constraint %s", rhs)
   573  	}).describef(obj, "validType(%s)", obj.Name())
   574  
   575  	aliasDecl := tdecl.Assign.IsValid()
   576  	if aliasDecl && tdecl.TypeParams.NumFields() != 0 {
   577  		// The parser will ensure this but we may still get an invalid AST.
   578  		// Complain and continue as regular type definition.
   579  		check.error(atPos(tdecl.Assign), BadDecl, "generic type cannot be alias")
   580  		aliasDecl = false
   581  	}
   582  
   583  	// alias declaration
   584  	if aliasDecl {
   585  		check.verifyVersionf(atPos(tdecl.Assign), go1_9, "type aliases")
   586  		if check.enableAlias {
   587  			// TODO(gri) Should be able to use nil instead of Typ[Invalid] to mark
   588  			//           the alias as incomplete. Currently this causes problems
   589  			//           with certain cycles. Investigate.
   590  			alias := check.newAlias(obj, Typ[Invalid])
   591  			setDefType(def, alias)
   592  			rhs = check.definedType(tdecl.Type, obj)
   593  			assert(rhs != nil)
   594  			alias.fromRHS = rhs
   595  			Unalias(alias) // resolve alias.actual
   596  		} else {
   597  			check.brokenAlias(obj)
   598  			rhs = check.typ(tdecl.Type)
   599  			check.validAlias(obj, rhs)
   600  		}
   601  		return
   602  	}
   603  
   604  	// type definition or generic type declaration
   605  	named := check.newNamed(obj, nil, nil)
   606  	setDefType(def, named)
   607  
   608  	if tdecl.TypeParams != nil {
   609  		check.openScope(tdecl, "type parameters")
   610  		defer check.closeScope()
   611  		check.collectTypeParams(&named.tparams, tdecl.TypeParams)
   612  	}
   613  
   614  	// determine underlying type of named
   615  	rhs = check.definedType(tdecl.Type, obj)
   616  	assert(rhs != nil)
   617  	named.fromRHS = rhs
   618  
   619  	// If the underlying type was not set while type-checking the right-hand
   620  	// side, it is invalid and an error should have been reported elsewhere.
   621  	if named.underlying == nil {
   622  		named.underlying = Typ[Invalid]
   623  	}
   624  
   625  	// Disallow a lone type parameter as the RHS of a type declaration (go.dev/issue/45639).
   626  	// We don't need this restriction anymore if we make the underlying type of a type
   627  	// parameter its constraint interface: if the RHS is a lone type parameter, we will
   628  	// use its underlying type (like we do for any RHS in a type declaration), and its
   629  	// underlying type is an interface and the type declaration is well defined.
   630  	if isTypeParam(rhs) {
   631  		check.error(tdecl.Type, MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
   632  		named.underlying = Typ[Invalid]
   633  	}
   634  }
   635  
   636  func (check *Checker) collectTypeParams(dst **TypeParamList, list *ast.FieldList) {
   637  	var tparams []*TypeParam
   638  	// Declare type parameters up-front, with empty interface as type bound.
   639  	// The scope of type parameters starts at the beginning of the type parameter
   640  	// list (so we can have mutually recursive parameterized interfaces).
   641  	scopePos := list.Pos()
   642  	for _, f := range list.List {
   643  		tparams = check.declareTypeParams(tparams, f.Names, scopePos)
   644  	}
   645  
   646  	// Set the type parameters before collecting the type constraints because
   647  	// the parameterized type may be used by the constraints (go.dev/issue/47887).
   648  	// Example: type T[P T[P]] interface{}
   649  	*dst = bindTParams(tparams)
   650  
   651  	// Signal to cycle detection that we are in a type parameter list.
   652  	// We can only be inside one type parameter list at any given time:
   653  	// function closures may appear inside a type parameter list but they
   654  	// cannot be generic, and their bodies are processed in delayed and
   655  	// sequential fashion. Note that with each new declaration, we save
   656  	// the existing environment and restore it when done; thus inTPList is
   657  	// true exactly only when we are in a specific type parameter list.
   658  	assert(!check.inTParamList)
   659  	check.inTParamList = true
   660  	defer func() {
   661  		check.inTParamList = false
   662  	}()
   663  
   664  	index := 0
   665  	for _, f := range list.List {
   666  		var bound Type
   667  		// NOTE: we may be able to assert that f.Type != nil here, but this is not
   668  		// an invariant of the AST, so we are cautious.
   669  		if f.Type != nil {
   670  			bound = check.bound(f.Type)
   671  			if isTypeParam(bound) {
   672  				// We may be able to allow this since it is now well-defined what
   673  				// the underlying type and thus type set of a type parameter is.
   674  				// But we may need some additional form of cycle detection within
   675  				// type parameter lists.
   676  				check.error(f.Type, MisplacedTypeParam, "cannot use a type parameter as constraint")
   677  				bound = Typ[Invalid]
   678  			}
   679  		} else {
   680  			bound = Typ[Invalid]
   681  		}
   682  		for i := range f.Names {
   683  			tparams[index+i].bound = bound
   684  		}
   685  		index += len(f.Names)
   686  	}
   687  }
   688  
   689  func (check *Checker) bound(x ast.Expr) Type {
   690  	// A type set literal of the form ~T and A|B may only appear as constraint;
   691  	// embed it in an implicit interface so that only interface type-checking
   692  	// needs to take care of such type expressions.
   693  	wrap := false
   694  	switch op := x.(type) {
   695  	case *ast.UnaryExpr:
   696  		wrap = op.Op == token.TILDE
   697  	case *ast.BinaryExpr:
   698  		wrap = op.Op == token.OR
   699  	}
   700  	if wrap {
   701  		x = &ast.InterfaceType{Methods: &ast.FieldList{List: []*ast.Field{{Type: x}}}}
   702  		t := check.typ(x)
   703  		// mark t as implicit interface if all went well
   704  		if t, _ := t.(*Interface); t != nil {
   705  			t.implicit = true
   706  		}
   707  		return t
   708  	}
   709  	return check.typ(x)
   710  }
   711  
   712  func (check *Checker) declareTypeParams(tparams []*TypeParam, names []*ast.Ident, scopePos token.Pos) []*TypeParam {
   713  	// Use Typ[Invalid] for the type constraint to ensure that a type
   714  	// is present even if the actual constraint has not been assigned
   715  	// yet.
   716  	// TODO(gri) Need to systematically review all uses of type parameter
   717  	//           constraints to make sure we don't rely on them if they
   718  	//           are not properly set yet.
   719  	for _, name := range names {
   720  		tname := NewTypeName(name.Pos(), check.pkg, name.Name, nil)
   721  		tpar := check.newTypeParam(tname, Typ[Invalid]) // assigns type to tpar as a side-effect
   722  		check.declare(check.scope, name, tname, scopePos)
   723  		tparams = append(tparams, tpar)
   724  	}
   725  
   726  	if check.conf._Trace && len(names) > 0 {
   727  		check.trace(names[0].Pos(), "type params = %v", tparams[len(tparams)-len(names):])
   728  	}
   729  
   730  	return tparams
   731  }
   732  
   733  func (check *Checker) collectMethods(obj *TypeName) {
   734  	// get associated methods
   735  	// (Checker.collectObjects only collects methods with non-blank names;
   736  	// Checker.resolveBaseTypeName ensures that obj is not an alias name
   737  	// if it has attached methods.)
   738  	methods := check.methods[obj]
   739  	if methods == nil {
   740  		return
   741  	}
   742  	delete(check.methods, obj)
   743  	assert(!check.objMap[obj].tdecl.Assign.IsValid()) // don't use TypeName.IsAlias (requires fully set up object)
   744  
   745  	// use an objset to check for name conflicts
   746  	var mset objset
   747  
   748  	// spec: "If the base type is a struct type, the non-blank method
   749  	// and field names must be distinct."
   750  	base := asNamed(obj.typ) // shouldn't fail but be conservative
   751  	if base != nil {
   752  		assert(base.TypeArgs().Len() == 0) // collectMethods should not be called on an instantiated type
   753  
   754  		// See go.dev/issue/52529: we must delay the expansion of underlying here, as
   755  		// base may not be fully set-up.
   756  		check.later(func() {
   757  			check.checkFieldUniqueness(base)
   758  		}).describef(obj, "verifying field uniqueness for %v", base)
   759  
   760  		// Checker.Files may be called multiple times; additional package files
   761  		// may add methods to already type-checked types. Add pre-existing methods
   762  		// so that we can detect redeclarations.
   763  		for i := 0; i < base.NumMethods(); i++ {
   764  			m := base.Method(i)
   765  			assert(m.name != "_")
   766  			assert(mset.insert(m) == nil)
   767  		}
   768  	}
   769  
   770  	// add valid methods
   771  	for _, m := range methods {
   772  		// spec: "For a base type, the non-blank names of methods bound
   773  		// to it must be unique."
   774  		assert(m.name != "_")
   775  		if alt := mset.insert(m); alt != nil {
   776  			if alt.Pos().IsValid() {
   777  				check.errorf(m, DuplicateMethod, "method %s.%s already declared at %s", obj.Name(), m.name, alt.Pos())
   778  			} else {
   779  				check.errorf(m, DuplicateMethod, "method %s.%s already declared", obj.Name(), m.name)
   780  			}
   781  			continue
   782  		}
   783  
   784  		if base != nil {
   785  			base.AddMethod(m)
   786  		}
   787  	}
   788  }
   789  
   790  func (check *Checker) checkFieldUniqueness(base *Named) {
   791  	if t, _ := base.under().(*Struct); t != nil {
   792  		var mset objset
   793  		for i := 0; i < base.NumMethods(); i++ {
   794  			m := base.Method(i)
   795  			assert(m.name != "_")
   796  			assert(mset.insert(m) == nil)
   797  		}
   798  
   799  		// Check that any non-blank field names of base are distinct from its
   800  		// method names.
   801  		for _, fld := range t.fields {
   802  			if fld.name != "_" {
   803  				if alt := mset.insert(fld); alt != nil {
   804  					// Struct fields should already be unique, so we should only
   805  					// encounter an alternate via collision with a method name.
   806  					_ = alt.(*Func)
   807  
   808  					// For historical consistency, we report the primary error on the
   809  					// method, and the alt decl on the field.
   810  					check.errorf(alt, DuplicateFieldAndMethod, "field and method with the same name %s", fld.name)
   811  					check.reportAltDecl(fld)
   812  				}
   813  			}
   814  		}
   815  	}
   816  }
   817  
   818  func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
   819  	assert(obj.typ == nil)
   820  
   821  	// func declarations cannot use iota
   822  	assert(check.iota == nil)
   823  
   824  	sig := new(Signature)
   825  	obj.typ = sig // guard against cycles
   826  
   827  	// Avoid cycle error when referring to method while type-checking the signature.
   828  	// This avoids a nuisance in the best case (non-parameterized receiver type) and
   829  	// since the method is not a type, we get an error. If we have a parameterized
   830  	// receiver type, instantiating the receiver type leads to the instantiation of
   831  	// its methods, and we don't want a cycle error in that case.
   832  	// TODO(gri) review if this is correct and/or whether we still need this?
   833  	saved := obj.color_
   834  	obj.color_ = black
   835  	fdecl := decl.fdecl
   836  	check.funcType(sig, fdecl.Recv, fdecl.Type)
   837  	obj.color_ = saved
   838  
   839  	// Set the scope's extent to the complete "func (...) { ... }"
   840  	// so that Scope.Innermost works correctly.
   841  	sig.scope.pos = fdecl.Pos()
   842  	sig.scope.end = fdecl.End()
   843  
   844  	if fdecl.Type.TypeParams.NumFields() > 0 && fdecl.Body == nil {
   845  		check.softErrorf(fdecl.Name, BadDecl, "generic function is missing function body")
   846  	}
   847  
   848  	// function body must be type-checked after global declarations
   849  	// (functions implemented elsewhere have no body)
   850  	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
   851  		check.later(func() {
   852  			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
   853  		}).describef(obj, "func %s", obj.name)
   854  	}
   855  }
   856  
   857  func (check *Checker) declStmt(d ast.Decl) {
   858  	pkg := check.pkg
   859  
   860  	check.walkDecl(d, func(d decl) {
   861  		switch d := d.(type) {
   862  		case constDecl:
   863  			top := len(check.delayed)
   864  
   865  			// declare all constants
   866  			lhs := make([]*Const, len(d.spec.Names))
   867  			for i, name := range d.spec.Names {
   868  				obj := NewConst(name.Pos(), pkg, name.Name, nil, constant.MakeInt64(int64(d.iota)))
   869  				lhs[i] = obj
   870  
   871  				var init ast.Expr
   872  				if i < len(d.init) {
   873  					init = d.init[i]
   874  				}
   875  
   876  				check.constDecl(obj, d.typ, init, d.inherited)
   877  			}
   878  
   879  			// process function literals in init expressions before scope changes
   880  			check.processDelayed(top)
   881  
   882  			// spec: "The scope of a constant or variable identifier declared
   883  			// inside a function begins at the end of the ConstSpec or VarSpec
   884  			// (ShortVarDecl for short variable declarations) and ends at the
   885  			// end of the innermost containing block."
   886  			scopePos := d.spec.End()
   887  			for i, name := range d.spec.Names {
   888  				check.declare(check.scope, name, lhs[i], scopePos)
   889  			}
   890  
   891  		case varDecl:
   892  			top := len(check.delayed)
   893  
   894  			lhs0 := make([]*Var, len(d.spec.Names))
   895  			for i, name := range d.spec.Names {
   896  				lhs0[i] = NewVar(name.Pos(), pkg, name.Name, nil)
   897  			}
   898  
   899  			// initialize all variables
   900  			for i, obj := range lhs0 {
   901  				var lhs []*Var
   902  				var init ast.Expr
   903  				switch len(d.spec.Values) {
   904  				case len(d.spec.Names):
   905  					// lhs and rhs match
   906  					init = d.spec.Values[i]
   907  				case 1:
   908  					// rhs is expected to be a multi-valued expression
   909  					lhs = lhs0
   910  					init = d.spec.Values[0]
   911  				default:
   912  					if i < len(d.spec.Values) {
   913  						init = d.spec.Values[i]
   914  					}
   915  				}
   916  				check.varDecl(obj, lhs, d.spec.Type, init)
   917  				if len(d.spec.Values) == 1 {
   918  					// If we have a single lhs variable we are done either way.
   919  					// If we have a single rhs expression, it must be a multi-
   920  					// valued expression, in which case handling the first lhs
   921  					// variable will cause all lhs variables to have a type
   922  					// assigned, and we are done as well.
   923  					if debug {
   924  						for _, obj := range lhs0 {
   925  							assert(obj.typ != nil)
   926  						}
   927  					}
   928  					break
   929  				}
   930  			}
   931  
   932  			// process function literals in init expressions before scope changes
   933  			check.processDelayed(top)
   934  
   935  			// declare all variables
   936  			// (only at this point are the variable scopes (parents) set)
   937  			scopePos := d.spec.End() // see constant declarations
   938  			for i, name := range d.spec.Names {
   939  				// see constant declarations
   940  				check.declare(check.scope, name, lhs0[i], scopePos)
   941  			}
   942  
   943  		case typeDecl:
   944  			obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
   945  			// spec: "The scope of a type identifier declared inside a function
   946  			// begins at the identifier in the TypeSpec and ends at the end of
   947  			// the innermost containing block."
   948  			scopePos := d.spec.Name.Pos()
   949  			check.declare(check.scope, d.spec.Name, obj, scopePos)
   950  			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
   951  			obj.setColor(grey + color(check.push(obj)))
   952  			check.typeDecl(obj, d.spec, nil)
   953  			check.pop().setColor(black)
   954  		default:
   955  			check.errorf(d.node(), InvalidSyntaxTree, "unknown ast.Decl node %T", d.node())
   956  		}
   957  	})
   958  }
   959  

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