Source file src/go/types/decl.go

     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/buildcfg"
    13  	. "internal/types/errors"
    14  )
    15  
    16  func (check *Checker) declare(scope *Scope, id *ast.Ident, obj Object, pos token.Pos) {
    17  	// spec: "The blank identifier, represented by the underscore
    18  	// character _, may be used in a declaration like any other
    19  	// identifier but the declaration does not introduce a new
    20  	// binding."
    21  	if obj.Name() != "_" {
    22  		if alt := scope.Insert(obj); alt != nil {
    23  			err := check.newError(DuplicateDecl)
    24  			err.addf(obj, "%s redeclared in this block", obj.Name())
    25  			err.addAltDecl(alt)
    26  			err.report()
    27  			return
    28  		}
    29  		obj.setScopePos(pos)
    30  	}
    31  	if id != nil {
    32  		check.recordDef(id, obj)
    33  	}
    34  }
    35  
    36  // pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
    37  func pathString(path []Object) string {
    38  	var s string
    39  	for i, p := range path {
    40  		if i > 0 {
    41  			s += "->"
    42  		}
    43  		s += p.Name()
    44  	}
    45  	return s
    46  }
    47  
    48  // objDecl type-checks the declaration of obj in its respective (file) environment.
    49  // For the meaning of def, see Checker.definedType, in typexpr.go.
    50  func (check *Checker) objDecl(obj Object, def *TypeName) {
    51  	if check.conf._Trace && obj.Type() == nil {
    52  		if check.indent == 0 {
    53  			fmt.Println() // empty line between top-level objects for readability
    54  		}
    55  		check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
    56  		check.indent++
    57  		defer func() {
    58  			check.indent--
    59  			check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
    60  		}()
    61  	}
    62  
    63  	// Checking the declaration of obj means inferring its type
    64  	// (and possibly its value, for constants).
    65  	// An object's type (and thus the object) may be in one of
    66  	// three states which are expressed by colors:
    67  	//
    68  	// - an object whose type is not yet known is painted white (initial color)
    69  	// - an object whose type is in the process of being inferred is painted grey
    70  	// - an object whose type is fully inferred is painted black
    71  	//
    72  	// During type inference, an object's color changes from white to grey
    73  	// to black (pre-declared objects are painted black from the start).
    74  	// A black object (i.e., its type) can only depend on (refer to) other black
    75  	// ones. White and grey objects may depend on white and black objects.
    76  	// A dependency on a grey object indicates a cycle which may or may not be
    77  	// valid.
    78  	//
    79  	// When objects turn grey, they are pushed on the object path (a stack);
    80  	// they are popped again when they turn black. Thus, if a grey object (a
    81  	// cycle) is encountered, it is on the object path, and all the objects
    82  	// it depends on are the remaining objects on that path. Color encoding
    83  	// is such that the color value of a grey object indicates the index of
    84  	// that object in the object path.
    85  
    86  	// During type-checking, white objects may be assigned a type without
    87  	// traversing through objDecl; e.g., when initializing constants and
    88  	// variables. Update the colors of those objects here (rather than
    89  	// everywhere where we set the type) to satisfy the color invariants.
    90  	if obj.color() == white && obj.Type() != nil {
    91  		obj.setColor(black)
    92  		return
    93  	}
    94  
    95  	switch obj.color() {
    96  	case white:
    97  		assert(obj.Type() == nil)
    98  		// All color values other than white and black are considered grey.
    99  		// Because black and white are < grey, all values >= grey are grey.
   100  		// Use those values to encode the object's index into the object path.
   101  		obj.setColor(grey + color(check.push(obj)))
   102  		defer func() {
   103  			check.pop().setColor(black)
   104  		}()
   105  
   106  	case black:
   107  		assert(obj.Type() != nil)
   108  		return
   109  
   110  	default:
   111  		// Color values other than white or black are considered grey.
   112  		fallthrough
   113  
   114  	case grey:
   115  		// We have a (possibly invalid) cycle.
   116  		// In the existing code, this is marked by a non-nil type
   117  		// for the object except for constants and variables whose
   118  		// type may be non-nil (known), or nil if it depends on the
   119  		// not-yet known initialization value.
   120  		// In the former case, set the type to Typ[Invalid] because
   121  		// we have an initialization cycle. The cycle error will be
   122  		// reported later, when determining initialization order.
   123  		// TODO(gri) Report cycle here and simplify initialization
   124  		// order code.
   125  		switch obj := obj.(type) {
   126  		case *Const:
   127  			if !check.validCycle(obj) || obj.typ == nil {
   128  				obj.typ = Typ[Invalid]
   129  			}
   130  
   131  		case *Var:
   132  			if !check.validCycle(obj) || obj.typ == nil {
   133  				obj.typ = Typ[Invalid]
   134  			}
   135  
   136  		case *TypeName:
   137  			if !check.validCycle(obj) {
   138  				// break cycle
   139  				// (without this, calling underlying()
   140  				// below may lead to an endless loop
   141  				// if we have a cycle for a defined
   142  				// (*Named) type)
   143  				obj.typ = Typ[Invalid]
   144  			}
   145  
   146  		case *Func:
   147  			if !check.validCycle(obj) {
   148  				// Don't set obj.typ to Typ[Invalid] here
   149  				// because plenty of code type-asserts that
   150  				// functions have a *Signature type. Grey
   151  				// functions have their type set to an empty
   152  				// signature which makes it impossible to
   153  				// initialize a variable with the function.
   154  			}
   155  
   156  		default:
   157  			panic("unreachable")
   158  		}
   159  		assert(obj.Type() != nil)
   160  		return
   161  	}
   162  
   163  	d := check.objMap[obj]
   164  	if d == nil {
   165  		check.dump("%v: %s should have been declared", obj.Pos(), obj)
   166  		panic("unreachable")
   167  	}
   168  
   169  	// save/restore current environment and set up object environment
   170  	defer func(env environment) {
   171  		check.environment = env
   172  	}(check.environment)
   173  	check.environment = environment{
   174  		scope: d.file,
   175  	}
   176  
   177  	// Const and var declarations must not have initialization
   178  	// cycles. We track them by remembering the current declaration
   179  	// in check.decl. Initialization expressions depending on other
   180  	// consts, vars, or functions, add dependencies to the current
   181  	// check.decl.
   182  	switch obj := obj.(type) {
   183  	case *Const:
   184  		check.decl = d // new package-level const decl
   185  		check.constDecl(obj, d.vtyp, d.init, d.inherited)
   186  	case *Var:
   187  		check.decl = d // new package-level var decl
   188  		check.varDecl(obj, d.lhs, d.vtyp, d.init)
   189  	case *TypeName:
   190  		// invalid recursive types are detected via path
   191  		check.typeDecl(obj, d.tdecl, def)
   192  		check.collectMethods(obj) // methods can only be added to top-level types
   193  	case *Func:
   194  		// functions may be recursive - no need to track dependencies
   195  		check.funcDecl(obj, d)
   196  	default:
   197  		panic("unreachable")
   198  	}
   199  }
   200  
   201  // validCycle checks if the cycle starting with obj is valid and
   202  // reports an error if it is not.
   203  func (check *Checker) validCycle(obj Object) (valid bool) {
   204  	// The object map contains the package scope objects and the non-interface methods.
   205  	if debug {
   206  		info := check.objMap[obj]
   207  		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
   208  		isPkgObj := obj.Parent() == check.pkg.scope
   209  		if isPkgObj != inObjMap {
   210  			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
   211  			panic("unreachable")
   212  		}
   213  	}
   214  
   215  	// Count cycle objects.
   216  	assert(obj.color() >= grey)
   217  	start := obj.color() - grey // index of obj in objPath
   218  	cycle := check.objPath[start:]
   219  	tparCycle := false // if set, the cycle is through a type parameter list
   220  	nval := 0          // number of (constant or variable) values in the cycle; valid if !generic
   221  	ndef := 0          // number of type definitions in the cycle; valid if !generic
   222  loop:
   223  	for _, obj := range cycle {
   224  		switch obj := obj.(type) {
   225  		case *Const, *Var:
   226  			nval++
   227  		case *TypeName:
   228  			// If we reach a generic type that is part of a cycle
   229  			// and we are in a type parameter list, we have a cycle
   230  			// through a type parameter list, which is invalid.
   231  			if check.inTParamList && isGeneric(obj.typ) {
   232  				tparCycle = true
   233  				break loop
   234  			}
   235  
   236  			// Determine if the type name is an alias or not. For
   237  			// package-level objects, use the object map which
   238  			// provides syntactic information (which doesn't rely
   239  			// on the order in which the objects are set up). For
   240  			// local objects, we can rely on the order, so use
   241  			// the object's predicate.
   242  			// TODO(gri) It would be less fragile to always access
   243  			// the syntactic information. We should consider storing
   244  			// this information explicitly in the object.
   245  			var alias bool
   246  			if check.conf._EnableAlias {
   247  				alias = obj.IsAlias()
   248  			} else {
   249  				if d := check.objMap[obj]; d != nil {
   250  					alias = d.tdecl.Assign.IsValid() // package-level object
   251  				} else {
   252  					alias = obj.IsAlias() // function local object
   253  				}
   254  			}
   255  			if !alias {
   256  				ndef++
   257  			}
   258  		case *Func:
   259  			// ignored for now
   260  		default:
   261  			panic("unreachable")
   262  		}
   263  	}
   264  
   265  	if check.conf._Trace {
   266  		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
   267  		if tparCycle {
   268  			check.trace(obj.Pos(), "## cycle contains: generic type in a type parameter list")
   269  		} else {
   270  			check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
   271  		}
   272  		defer func() {
   273  			if valid {
   274  				check.trace(obj.Pos(), "=> cycle is valid")
   275  			} else {
   276  				check.trace(obj.Pos(), "=> error: cycle is invalid")
   277  			}
   278  		}()
   279  	}
   280  
   281  	if !tparCycle {
   282  		// A cycle involving only constants and variables is invalid but we
   283  		// ignore them here because they are reported via the initialization
   284  		// cycle check.
   285  		if nval == len(cycle) {
   286  			return true
   287  		}
   288  
   289  		// A cycle involving only types (and possibly functions) must have at least
   290  		// one type definition to be permitted: If there is no type definition, we
   291  		// have a sequence of alias type names which will expand ad infinitum.
   292  		if nval == 0 && ndef > 0 {
   293  			return true
   294  		}
   295  	}
   296  
   297  	check.cycleError(cycle, firstInSrc(cycle))
   298  	return false
   299  }
   300  
   301  // cycleError reports a declaration cycle starting with the object at cycle[start].
   302  func (check *Checker) cycleError(cycle []Object, start int) {
   303  	// name returns the (possibly qualified) object name.
   304  	// This is needed because with generic types, cycles
   305  	// may refer to imported types. See go.dev/issue/50788.
   306  	// TODO(gri) Thus functionality is used elsewhere. Factor it out.
   307  	name := func(obj Object) string {
   308  		return packagePrefix(obj.Pkg(), check.qualifier) + obj.Name()
   309  	}
   310  
   311  	obj := cycle[start]
   312  	objName := name(obj)
   313  	// If obj is a type alias, mark it as valid (not broken) in order to avoid follow-on errors.
   314  	tname, _ := obj.(*TypeName)
   315  	if tname != nil && tname.IsAlias() {
   316  		// If we use Alias nodes, it is initialized with Typ[Invalid].
   317  		// TODO(gri) Adjust this code if we initialize with nil.
   318  		if !check.conf._EnableAlias {
   319  			check.validAlias(tname, Typ[Invalid])
   320  		}
   321  	}
   322  
   323  	// report a more concise error for self references
   324  	if len(cycle) == 1 {
   325  		if tname != nil {
   326  			check.errorf(obj, InvalidDeclCycle, "invalid recursive type: %s refers to itself", objName)
   327  		} else {
   328  			check.errorf(obj, InvalidDeclCycle, "invalid cycle in declaration: %s refers to itself", objName)
   329  		}
   330  		return
   331  	}
   332  
   333  	err := check.newError(InvalidDeclCycle)
   334  	if tname != nil {
   335  		err.addf(obj, "invalid recursive type %s", objName)
   336  	} else {
   337  		err.addf(obj, "invalid cycle in declaration of %s", objName)
   338  	}
   339  	i := start
   340  	for range cycle {
   341  		err.addf(obj, "%s refers to", objName)
   342  		i++
   343  		if i >= len(cycle) {
   344  			i = 0
   345  		}
   346  		obj = cycle[i]
   347  		objName = name(obj)
   348  	}
   349  	err.addf(obj, "%s", objName)
   350  	err.report()
   351  }
   352  
   353  // firstInSrc reports the index of the object with the "smallest"
   354  // source position in path. path must not be empty.
   355  func firstInSrc(path []Object) int {
   356  	fst, pos := 0, path[0].Pos()
   357  	for i, t := range path[1:] {
   358  		if cmpPos(t.Pos(), pos) < 0 {
   359  			fst, pos = i+1, t.Pos()
   360  		}
   361  	}
   362  	return fst
   363  }
   364  
   365  type (
   366  	decl interface {
   367  		node() ast.Node
   368  	}
   369  
   370  	importDecl struct{ spec *ast.ImportSpec }
   371  	constDecl  struct {
   372  		spec      *ast.ValueSpec
   373  		iota      int
   374  		typ       ast.Expr
   375  		init      []ast.Expr
   376  		inherited bool
   377  	}
   378  	varDecl  struct{ spec *ast.ValueSpec }
   379  	typeDecl struct{ spec *ast.TypeSpec }
   380  	funcDecl struct{ decl *ast.FuncDecl }
   381  )
   382  
   383  func (d importDecl) node() ast.Node { return d.spec }
   384  func (d constDecl) node() ast.Node  { return d.spec }
   385  func (d varDecl) node() ast.Node    { return d.spec }
   386  func (d typeDecl) node() ast.Node   { return d.spec }
   387  func (d funcDecl) node() ast.Node   { return d.decl }
   388  
   389  func (check *Checker) walkDecls(decls []ast.Decl, f func(decl)) {
   390  	for _, d := range decls {
   391  		check.walkDecl(d, f)
   392  	}
   393  }
   394  
   395  func (check *Checker) walkDecl(d ast.Decl, f func(decl)) {
   396  	switch d := d.(type) {
   397  	case *ast.BadDecl:
   398  		// ignore
   399  	case *ast.GenDecl:
   400  		var last *ast.ValueSpec // last ValueSpec with type or init exprs seen
   401  		for iota, s := range d.Specs {
   402  			switch s := s.(type) {
   403  			case *ast.ImportSpec:
   404  				f(importDecl{s})
   405  			case *ast.ValueSpec:
   406  				switch d.Tok {
   407  				case token.CONST:
   408  					// determine which initialization expressions to use
   409  					inherited := true
   410  					switch {
   411  					case s.Type != nil || len(s.Values) > 0:
   412  						last = s
   413  						inherited = false
   414  					case last == nil:
   415  						last = new(ast.ValueSpec) // make sure last exists
   416  						inherited = false
   417  					}
   418  					check.arityMatch(s, last)
   419  					f(constDecl{spec: s, iota: iota, typ: last.Type, init: last.Values, inherited: inherited})
   420  				case token.VAR:
   421  					check.arityMatch(s, nil)
   422  					f(varDecl{s})
   423  				default:
   424  					check.errorf(s, InvalidSyntaxTree, "invalid token %s", d.Tok)
   425  				}
   426  			case *ast.TypeSpec:
   427  				f(typeDecl{s})
   428  			default:
   429  				check.errorf(s, InvalidSyntaxTree, "unknown ast.Spec node %T", s)
   430  			}
   431  		}
   432  	case *ast.FuncDecl:
   433  		f(funcDecl{d})
   434  	default:
   435  		check.errorf(d, InvalidSyntaxTree, "unknown ast.Decl node %T", d)
   436  	}
   437  }
   438  
   439  func (check *Checker) constDecl(obj *Const, typ, init ast.Expr, inherited bool) {
   440  	assert(obj.typ == nil)
   441  
   442  	// use the correct value of iota
   443  	defer func(iota constant.Value, errpos positioner) {
   444  		check.iota = iota
   445  		check.errpos = errpos
   446  	}(check.iota, check.errpos)
   447  	check.iota = obj.val
   448  	check.errpos = nil
   449  
   450  	// provide valid constant value under all circumstances
   451  	obj.val = constant.MakeUnknown()
   452  
   453  	// determine type, if any
   454  	if typ != nil {
   455  		t := check.typ(typ)
   456  		if !isConstType(t) {
   457  			// don't report an error if the type is an invalid C (defined) type
   458  			// (go.dev/issue/22090)
   459  			if isValid(under(t)) {
   460  				check.errorf(typ, InvalidConstType, "invalid constant type %s", t)
   461  			}
   462  			obj.typ = Typ[Invalid]
   463  			return
   464  		}
   465  		obj.typ = t
   466  	}
   467  
   468  	// check initialization
   469  	var x operand
   470  	if init != nil {
   471  		if inherited {
   472  			// The initialization expression is inherited from a previous
   473  			// constant declaration, and (error) positions refer to that
   474  			// expression and not the current constant declaration. Use
   475  			// the constant identifier position for any errors during
   476  			// init expression evaluation since that is all we have
   477  			// (see issues go.dev/issue/42991, go.dev/issue/42992).
   478  			check.errpos = atPos(obj.pos)
   479  		}
   480  		check.expr(nil, &x, init)
   481  	}
   482  	check.initConst(obj, &x)
   483  }
   484  
   485  func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
   486  	assert(obj.typ == nil)
   487  
   488  	// determine type, if any
   489  	if typ != nil {
   490  		obj.typ = check.varType(typ)
   491  		// We cannot spread the type to all lhs variables if there
   492  		// are more than one since that would mark them as checked
   493  		// (see Checker.objDecl) and the assignment of init exprs,
   494  		// if any, would not be checked.
   495  		//
   496  		// TODO(gri) If we have no init expr, we should distribute
   497  		// a given type otherwise we need to re-evaluate the type
   498  		// expr for each lhs variable, leading to duplicate work.
   499  	}
   500  
   501  	// check initialization
   502  	if init == nil {
   503  		if typ == nil {
   504  			// error reported before by arityMatch
   505  			obj.typ = Typ[Invalid]
   506  		}
   507  		return
   508  	}
   509  
   510  	if lhs == nil || len(lhs) == 1 {
   511  		assert(lhs == nil || lhs[0] == obj)
   512  		var x operand
   513  		check.expr(newTarget(obj.typ, obj.name), &x, init)
   514  		check.initVar(obj, &x, "variable declaration")
   515  		return
   516  	}
   517  
   518  	if debug {
   519  		// obj must be one of lhs
   520  		found := false
   521  		for _, lhs := range lhs {
   522  			if obj == lhs {
   523  				found = true
   524  				break
   525  			}
   526  		}
   527  		if !found {
   528  			panic("inconsistent lhs")
   529  		}
   530  	}
   531  
   532  	// We have multiple variables on the lhs and one init expr.
   533  	// Make sure all variables have been given the same type if
   534  	// one was specified, otherwise they assume the type of the
   535  	// init expression values (was go.dev/issue/15755).
   536  	if typ != nil {
   537  		for _, lhs := range lhs {
   538  			lhs.typ = obj.typ
   539  		}
   540  	}
   541  
   542  	check.initVars(lhs, []ast.Expr{init}, nil)
   543  }
   544  
   545  // isImportedConstraint reports whether typ is an imported type constraint.
   546  func (check *Checker) isImportedConstraint(typ Type) bool {
   547  	named := asNamed(typ)
   548  	if named == nil || named.obj.pkg == check.pkg || named.obj.pkg == nil {
   549  		return false
   550  	}
   551  	u, _ := named.under().(*Interface)
   552  	return u != nil && !u.IsMethodSet()
   553  }
   554  
   555  func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName) {
   556  	assert(obj.typ == nil)
   557  
   558  	// Only report a version error if we have not reported one already.
   559  	versionErr := false
   560  
   561  	var rhs Type
   562  	check.later(func() {
   563  		if t := asNamed(obj.typ); t != nil { // type may be invalid
   564  			check.validType(t)
   565  		}
   566  		// If typ is local, an error was already reported where typ is specified/defined.
   567  		_ = !versionErr && check.isImportedConstraint(rhs) && check.verifyVersionf(tdecl.Type, go1_18, "using type constraint %s", rhs)
   568  	}).describef(obj, "validType(%s)", obj.Name())
   569  
   570  	// First type parameter, or nil.
   571  	var tparam0 *ast.Field
   572  	if tdecl.TypeParams.NumFields() > 0 {
   573  		tparam0 = tdecl.TypeParams.List[0]
   574  	}
   575  
   576  	// alias declaration
   577  	if tdecl.Assign.IsValid() {
   578  		// Report highest version requirement first so that fixing a version issue
   579  		// avoids possibly two -lang changes (first to Go 1.9 and then to Go 1.23).
   580  		if !versionErr && tparam0 != nil && !check.verifyVersionf(tparam0, go1_23, "generic type alias") {
   581  			versionErr = true
   582  		}
   583  		if !versionErr && !check.verifyVersionf(atPos(tdecl.Assign), go1_9, "type alias") {
   584  			versionErr = true
   585  		}
   586  
   587  		if check.conf._EnableAlias {
   588  			// TODO(gri) Should be able to use nil instead of Typ[Invalid] to mark
   589  			//           the alias as incomplete. Currently this causes problems
   590  			//           with certain cycles. Investigate.
   591  			//
   592  			// NOTE(adonovan): to avoid the Invalid being prematurely observed
   593  			// by (e.g.) a var whose type is an unfinished cycle,
   594  			// Unalias does not memoize if Invalid. Perhaps we should use a
   595  			// special sentinel distinct from Invalid.
   596  			alias := check.newAlias(obj, Typ[Invalid])
   597  			setDefType(def, alias)
   598  
   599  			// handle type parameters even if not allowed (Alias type is supported)
   600  			if tparam0 != nil {
   601  				if !versionErr && !buildcfg.Experiment.AliasTypeParams {
   602  					check.error(tdecl, UnsupportedFeature, "generic type alias requires GOEXPERIMENT=aliastypeparams")
   603  					versionErr = true
   604  				}
   605  				check.openScope(tdecl, "type parameters")
   606  				defer check.closeScope()
   607  				check.collectTypeParams(&alias.tparams, tdecl.TypeParams)
   608  			}
   609  
   610  			rhs = check.definedType(tdecl.Type, obj)
   611  			assert(rhs != nil)
   612  			alias.fromRHS = rhs
   613  			Unalias(alias) // resolve alias.actual
   614  		} else {
   615  			// With Go1.23, the default behavior is to use Alias nodes,
   616  			// reflected by check.enableAlias. Signal non-default behavior.
   617  			//
   618  			// TODO(gri) Testing runs tests in both modes. Do we need to exclude
   619  			//           tracking of non-default behavior for tests?
   620  			gotypesalias.IncNonDefault()
   621  
   622  			if !versionErr && tparam0 != nil {
   623  				check.error(tdecl, UnsupportedFeature, "generic type alias requires GODEBUG=gotypesalias=1 or unset")
   624  				versionErr = true
   625  			}
   626  
   627  			check.brokenAlias(obj)
   628  			rhs = check.typ(tdecl.Type)
   629  			check.validAlias(obj, rhs)
   630  		}
   631  		return
   632  	}
   633  
   634  	// type definition or generic type declaration
   635  	if !versionErr && tparam0 != nil && !check.verifyVersionf(tparam0, go1_18, "type parameter") {
   636  		versionErr = true
   637  	}
   638  
   639  	named := check.newNamed(obj, nil, nil)
   640  	setDefType(def, named)
   641  
   642  	if tdecl.TypeParams != nil {
   643  		check.openScope(tdecl, "type parameters")
   644  		defer check.closeScope()
   645  		check.collectTypeParams(&named.tparams, tdecl.TypeParams)
   646  	}
   647  
   648  	// determine underlying type of named
   649  	rhs = check.definedType(tdecl.Type, obj)
   650  	assert(rhs != nil)
   651  	named.fromRHS = rhs
   652  
   653  	// If the underlying type was not set while type-checking the right-hand
   654  	// side, it is invalid and an error should have been reported elsewhere.
   655  	if named.underlying == nil {
   656  		named.underlying = Typ[Invalid]
   657  	}
   658  
   659  	// Disallow a lone type parameter as the RHS of a type declaration (go.dev/issue/45639).
   660  	// We don't need this restriction anymore if we make the underlying type of a type
   661  	// parameter its constraint interface: if the RHS is a lone type parameter, we will
   662  	// use its underlying type (like we do for any RHS in a type declaration), and its
   663  	// underlying type is an interface and the type declaration is well defined.
   664  	if isTypeParam(rhs) {
   665  		check.error(tdecl.Type, MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
   666  		named.underlying = Typ[Invalid]
   667  	}
   668  }
   669  
   670  func (check *Checker) collectTypeParams(dst **TypeParamList, list *ast.FieldList) {
   671  	var tparams []*TypeParam
   672  	// Declare type parameters up-front, with empty interface as type bound.
   673  	// The scope of type parameters starts at the beginning of the type parameter
   674  	// list (so we can have mutually recursive parameterized interfaces).
   675  	scopePos := list.Pos()
   676  	for _, f := range list.List {
   677  		tparams = check.declareTypeParams(tparams, f.Names, scopePos)
   678  	}
   679  
   680  	// Set the type parameters before collecting the type constraints because
   681  	// the parameterized type may be used by the constraints (go.dev/issue/47887).
   682  	// Example: type T[P T[P]] interface{}
   683  	*dst = bindTParams(tparams)
   684  
   685  	// Signal to cycle detection that we are in a type parameter list.
   686  	// We can only be inside one type parameter list at any given time:
   687  	// function closures may appear inside a type parameter list but they
   688  	// cannot be generic, and their bodies are processed in delayed and
   689  	// sequential fashion. Note that with each new declaration, we save
   690  	// the existing environment and restore it when done; thus inTPList is
   691  	// true exactly only when we are in a specific type parameter list.
   692  	assert(!check.inTParamList)
   693  	check.inTParamList = true
   694  	defer func() {
   695  		check.inTParamList = false
   696  	}()
   697  
   698  	index := 0
   699  	for _, f := range list.List {
   700  		var bound Type
   701  		// NOTE: we may be able to assert that f.Type != nil here, but this is not
   702  		// an invariant of the AST, so we are cautious.
   703  		if f.Type != nil {
   704  			bound = check.bound(f.Type)
   705  			if isTypeParam(bound) {
   706  				// We may be able to allow this since it is now well-defined what
   707  				// the underlying type and thus type set of a type parameter is.
   708  				// But we may need some additional form of cycle detection within
   709  				// type parameter lists.
   710  				check.error(f.Type, MisplacedTypeParam, "cannot use a type parameter as constraint")
   711  				bound = Typ[Invalid]
   712  			}
   713  		} else {
   714  			bound = Typ[Invalid]
   715  		}
   716  		for i := range f.Names {
   717  			tparams[index+i].bound = bound
   718  		}
   719  		index += len(f.Names)
   720  	}
   721  }
   722  
   723  func (check *Checker) bound(x ast.Expr) Type {
   724  	// A type set literal of the form ~T and A|B may only appear as constraint;
   725  	// embed it in an implicit interface so that only interface type-checking
   726  	// needs to take care of such type expressions.
   727  	wrap := false
   728  	switch op := x.(type) {
   729  	case *ast.UnaryExpr:
   730  		wrap = op.Op == token.TILDE
   731  	case *ast.BinaryExpr:
   732  		wrap = op.Op == token.OR
   733  	}
   734  	if wrap {
   735  		x = &ast.InterfaceType{Methods: &ast.FieldList{List: []*ast.Field{{Type: x}}}}
   736  		t := check.typ(x)
   737  		// mark t as implicit interface if all went well
   738  		if t, _ := t.(*Interface); t != nil {
   739  			t.implicit = true
   740  		}
   741  		return t
   742  	}
   743  	return check.typ(x)
   744  }
   745  
   746  func (check *Checker) declareTypeParams(tparams []*TypeParam, names []*ast.Ident, scopePos token.Pos) []*TypeParam {
   747  	// Use Typ[Invalid] for the type constraint to ensure that a type
   748  	// is present even if the actual constraint has not been assigned
   749  	// yet.
   750  	// TODO(gri) Need to systematically review all uses of type parameter
   751  	//           constraints to make sure we don't rely on them if they
   752  	//           are not properly set yet.
   753  	for _, name := range names {
   754  		tname := NewTypeName(name.Pos(), check.pkg, name.Name, nil)
   755  		tpar := check.newTypeParam(tname, Typ[Invalid]) // assigns type to tpar as a side-effect
   756  		check.declare(check.scope, name, tname, scopePos)
   757  		tparams = append(tparams, tpar)
   758  	}
   759  
   760  	if check.conf._Trace && len(names) > 0 {
   761  		check.trace(names[0].Pos(), "type params = %v", tparams[len(tparams)-len(names):])
   762  	}
   763  
   764  	return tparams
   765  }
   766  
   767  func (check *Checker) collectMethods(obj *TypeName) {
   768  	// get associated methods
   769  	// (Checker.collectObjects only collects methods with non-blank names;
   770  	// Checker.resolveBaseTypeName ensures that obj is not an alias name
   771  	// if it has attached methods.)
   772  	methods := check.methods[obj]
   773  	if methods == nil {
   774  		return
   775  	}
   776  	delete(check.methods, obj)
   777  	assert(!check.objMap[obj].tdecl.Assign.IsValid()) // don't use TypeName.IsAlias (requires fully set up object)
   778  
   779  	// use an objset to check for name conflicts
   780  	var mset objset
   781  
   782  	// spec: "If the base type is a struct type, the non-blank method
   783  	// and field names must be distinct."
   784  	base := asNamed(obj.typ) // shouldn't fail but be conservative
   785  	if base != nil {
   786  		assert(base.TypeArgs().Len() == 0) // collectMethods should not be called on an instantiated type
   787  
   788  		// See go.dev/issue/52529: we must delay the expansion of underlying here, as
   789  		// base may not be fully set-up.
   790  		check.later(func() {
   791  			check.checkFieldUniqueness(base)
   792  		}).describef(obj, "verifying field uniqueness for %v", base)
   793  
   794  		// Checker.Files may be called multiple times; additional package files
   795  		// may add methods to already type-checked types. Add pre-existing methods
   796  		// so that we can detect redeclarations.
   797  		for i := 0; i < base.NumMethods(); i++ {
   798  			m := base.Method(i)
   799  			assert(m.name != "_")
   800  			assert(mset.insert(m) == nil)
   801  		}
   802  	}
   803  
   804  	// add valid methods
   805  	for _, m := range methods {
   806  		// spec: "For a base type, the non-blank names of methods bound
   807  		// to it must be unique."
   808  		assert(m.name != "_")
   809  		if alt := mset.insert(m); alt != nil {
   810  			if alt.Pos().IsValid() {
   811  				check.errorf(m, DuplicateMethod, "method %s.%s already declared at %v", obj.Name(), m.name, alt.Pos())
   812  			} else {
   813  				check.errorf(m, DuplicateMethod, "method %s.%s already declared", obj.Name(), m.name)
   814  			}
   815  			continue
   816  		}
   817  
   818  		if base != nil {
   819  			base.AddMethod(m)
   820  		}
   821  	}
   822  }
   823  
   824  func (check *Checker) checkFieldUniqueness(base *Named) {
   825  	if t, _ := base.under().(*Struct); t != nil {
   826  		var mset objset
   827  		for i := 0; i < base.NumMethods(); i++ {
   828  			m := base.Method(i)
   829  			assert(m.name != "_")
   830  			assert(mset.insert(m) == nil)
   831  		}
   832  
   833  		// Check that any non-blank field names of base are distinct from its
   834  		// method names.
   835  		for _, fld := range t.fields {
   836  			if fld.name != "_" {
   837  				if alt := mset.insert(fld); alt != nil {
   838  					// Struct fields should already be unique, so we should only
   839  					// encounter an alternate via collision with a method name.
   840  					_ = alt.(*Func)
   841  
   842  					// For historical consistency, we report the primary error on the
   843  					// method, and the alt decl on the field.
   844  					err := check.newError(DuplicateFieldAndMethod)
   845  					err.addf(alt, "field and method with the same name %s", fld.name)
   846  					err.addAltDecl(fld)
   847  					err.report()
   848  				}
   849  			}
   850  		}
   851  	}
   852  }
   853  
   854  func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
   855  	assert(obj.typ == nil)
   856  
   857  	// func declarations cannot use iota
   858  	assert(check.iota == nil)
   859  
   860  	sig := new(Signature)
   861  	obj.typ = sig // guard against cycles
   862  
   863  	// Avoid cycle error when referring to method while type-checking the signature.
   864  	// This avoids a nuisance in the best case (non-parameterized receiver type) and
   865  	// since the method is not a type, we get an error. If we have a parameterized
   866  	// receiver type, instantiating the receiver type leads to the instantiation of
   867  	// its methods, and we don't want a cycle error in that case.
   868  	// TODO(gri) review if this is correct and/or whether we still need this?
   869  	saved := obj.color_
   870  	obj.color_ = black
   871  	fdecl := decl.fdecl
   872  	check.funcType(sig, fdecl.Recv, fdecl.Type)
   873  	obj.color_ = saved
   874  
   875  	// Set the scope's extent to the complete "func (...) { ... }"
   876  	// so that Scope.Innermost works correctly.
   877  	sig.scope.pos = fdecl.Pos()
   878  	sig.scope.end = fdecl.End()
   879  
   880  	if fdecl.Type.TypeParams.NumFields() > 0 && fdecl.Body == nil {
   881  		check.softErrorf(fdecl.Name, BadDecl, "generic function is missing function body")
   882  	}
   883  
   884  	// function body must be type-checked after global declarations
   885  	// (functions implemented elsewhere have no body)
   886  	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
   887  		check.later(func() {
   888  			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
   889  		}).describef(obj, "func %s", obj.name)
   890  	}
   891  }
   892  
   893  func (check *Checker) declStmt(d ast.Decl) {
   894  	pkg := check.pkg
   895  
   896  	check.walkDecl(d, func(d decl) {
   897  		switch d := d.(type) {
   898  		case constDecl:
   899  			top := len(check.delayed)
   900  
   901  			// declare all constants
   902  			lhs := make([]*Const, len(d.spec.Names))
   903  			for i, name := range d.spec.Names {
   904  				obj := NewConst(name.Pos(), pkg, name.Name, nil, constant.MakeInt64(int64(d.iota)))
   905  				lhs[i] = obj
   906  
   907  				var init ast.Expr
   908  				if i < len(d.init) {
   909  					init = d.init[i]
   910  				}
   911  
   912  				check.constDecl(obj, d.typ, init, d.inherited)
   913  			}
   914  
   915  			// process function literals in init expressions before scope changes
   916  			check.processDelayed(top)
   917  
   918  			// spec: "The scope of a constant or variable identifier declared
   919  			// inside a function begins at the end of the ConstSpec or VarSpec
   920  			// (ShortVarDecl for short variable declarations) and ends at the
   921  			// end of the innermost containing block."
   922  			scopePos := d.spec.End()
   923  			for i, name := range d.spec.Names {
   924  				check.declare(check.scope, name, lhs[i], scopePos)
   925  			}
   926  
   927  		case varDecl:
   928  			top := len(check.delayed)
   929  
   930  			lhs0 := make([]*Var, len(d.spec.Names))
   931  			for i, name := range d.spec.Names {
   932  				lhs0[i] = NewVar(name.Pos(), pkg, name.Name, nil)
   933  			}
   934  
   935  			// initialize all variables
   936  			for i, obj := range lhs0 {
   937  				var lhs []*Var
   938  				var init ast.Expr
   939  				switch len(d.spec.Values) {
   940  				case len(d.spec.Names):
   941  					// lhs and rhs match
   942  					init = d.spec.Values[i]
   943  				case 1:
   944  					// rhs is expected to be a multi-valued expression
   945  					lhs = lhs0
   946  					init = d.spec.Values[0]
   947  				default:
   948  					if i < len(d.spec.Values) {
   949  						init = d.spec.Values[i]
   950  					}
   951  				}
   952  				check.varDecl(obj, lhs, d.spec.Type, init)
   953  				if len(d.spec.Values) == 1 {
   954  					// If we have a single lhs variable we are done either way.
   955  					// If we have a single rhs expression, it must be a multi-
   956  					// valued expression, in which case handling the first lhs
   957  					// variable will cause all lhs variables to have a type
   958  					// assigned, and we are done as well.
   959  					if debug {
   960  						for _, obj := range lhs0 {
   961  							assert(obj.typ != nil)
   962  						}
   963  					}
   964  					break
   965  				}
   966  			}
   967  
   968  			// process function literals in init expressions before scope changes
   969  			check.processDelayed(top)
   970  
   971  			// declare all variables
   972  			// (only at this point are the variable scopes (parents) set)
   973  			scopePos := d.spec.End() // see constant declarations
   974  			for i, name := range d.spec.Names {
   975  				// see constant declarations
   976  				check.declare(check.scope, name, lhs0[i], scopePos)
   977  			}
   978  
   979  		case typeDecl:
   980  			obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
   981  			// spec: "The scope of a type identifier declared inside a function
   982  			// begins at the identifier in the TypeSpec and ends at the end of
   983  			// the innermost containing block."
   984  			scopePos := d.spec.Name.Pos()
   985  			check.declare(check.scope, d.spec.Name, obj, scopePos)
   986  			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
   987  			obj.setColor(grey + color(check.push(obj)))
   988  			check.typeDecl(obj, d.spec, nil)
   989  			check.pop().setColor(black)
   990  		default:
   991  			check.errorf(d.node(), InvalidSyntaxTree, "unknown ast.Decl node %T", d.node())
   992  		}
   993  	})
   994  }
   995  

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