go/types, types2: introduce _Alias type node

[gostls13.git] / src / go / types / decl.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package types

import (
        "fmt"
        "go/ast"
        "go/constant"
        "go/token"
        . "internal/types/errors"
)

func (check *Checker) reportAltDecl(obj Object) {
        if pos := obj.Pos(); pos.IsValid() {
                // We use "other" rather than "previous" here because
                // the first declaration seen may not be textually
                // earlier in the source.
                check.errorf(obj, DuplicateDecl, "\tother declaration of %s", obj.Name()) // secondary error, \t indented
        }
}

func (check *Checker) declare(scope *Scope, id *ast.Ident, obj Object, pos token.Pos) {
        // spec: "The blank identifier, represented by the underscore
        // character _, may be used in a declaration like any other
        // identifier but the declaration does not introduce a new
        // binding."
        if obj.Name() != "_" {
                if alt := scope.Insert(obj); alt != nil {
                        check.errorf(obj, DuplicateDecl, "%s redeclared in this block", obj.Name())
                        check.reportAltDecl(alt)
                        return
                }
                obj.setScopePos(pos)
        }
        if id != nil {
                check.recordDef(id, obj)
        }
}

// pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
func pathString(path []Object) string {
        var s string
        for i, p := range path {
                if i > 0 {
                        s += "->"
                }
                s += p.Name()
        }
        return s
}

// objDecl type-checks the declaration of obj in its respective (file) environment.
// For the meaning of def, see Checker.definedType, in typexpr.go.
func (check *Checker) objDecl(obj Object, def *TypeName) {
        if check.conf._Trace && obj.Type() == nil {
                if check.indent == 0 {
                        fmt.Println() // empty line between top-level objects for readability
                }
                check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
                check.indent++
                defer func() {
                        check.indent--
                        check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
                }()
        }

        // Checking the declaration of obj means inferring its type
        // (and possibly its value, for constants).
        // An object's type (and thus the object) may be in one of
        // three states which are expressed by colors:
        //
        // - an object whose type is not yet known is painted white (initial color)
        // - an object whose type is in the process of being inferred is painted grey
        // - an object whose type is fully inferred is painted black
        //
        // During type inference, an object's color changes from white to grey
        // to black (pre-declared objects are painted black from the start).
        // A black object (i.e., its type) can only depend on (refer to) other black
        // ones. White and grey objects may depend on white and black objects.
        // A dependency on a grey object indicates a cycle which may or may not be
        // valid.
        //
        // When objects turn grey, they are pushed on the object path (a stack);
        // they are popped again when they turn black. Thus, if a grey object (a
        // cycle) is encountered, it is on the object path, and all the objects
        // it depends on are the remaining objects on that path. Color encoding
        // is such that the color value of a grey object indicates the index of
        // that object in the object path.

        // During type-checking, white objects may be assigned a type without
        // traversing through objDecl; e.g., when initializing constants and
        // variables. Update the colors of those objects here (rather than
        // everywhere where we set the type) to satisfy the color invariants.
        if obj.color() == white && obj.Type() != nil {
                obj.setColor(black)
                return
        }

        switch obj.color() {
        case white:
                assert(obj.Type() == nil)
                // All color values other than white and black are considered grey.
                // Because black and white are < grey, all values >= grey are grey.
                // Use those values to encode the object's index into the object path.
                obj.setColor(grey + color(check.push(obj)))
                defer func() {
                        check.pop().setColor(black)
                }()

        case black:
                assert(obj.Type() != nil)
                return

        default:
                // Color values other than white or black are considered grey.
                fallthrough

        case grey:
                // We have a (possibly invalid) cycle.
                // In the existing code, this is marked by a non-nil type
                // for the object except for constants and variables whose
                // type may be non-nil (known), or nil if it depends on the
                // not-yet known initialization value.
                // In the former case, set the type to Typ[Invalid] because
                // we have an initialization cycle. The cycle error will be
                // reported later, when determining initialization order.
                // TODO(gri) Report cycle here and simplify initialization
                // order code.
                switch obj := obj.(type) {
                case *Const:
                        if !check.validCycle(obj) || obj.typ == nil {
                                obj.typ = Typ[Invalid]
                        }

                case *Var:
                        if !check.validCycle(obj) || obj.typ == nil {
                                obj.typ = Typ[Invalid]
                        }

                case *TypeName:
                        if !check.validCycle(obj) {
                                // break cycle
                                // (without this, calling underlying()
                                // below may lead to an endless loop
                                // if we have a cycle for a defined
                                // (*Named) type)
                                obj.typ = Typ[Invalid]
                        }

                case *Func:
                        if !check.validCycle(obj) {
                                // Don't set obj.typ to Typ[Invalid] here
                                // because plenty of code type-asserts that
                                // functions have a *Signature type. Grey
                                // functions have their type set to an empty
                                // signature which makes it impossible to
                                // initialize a variable with the function.
                        }

                default:
                        unreachable()
                }
                assert(obj.Type() != nil)
                return
        }

        d := check.objMap[obj]
        if d == nil {
                check.dump("%v: %s should have been declared", obj.Pos(), obj)
                unreachable()
        }

        // save/restore current environment and set up object environment
        defer func(env environment) {
                check.environment = env
        }(check.environment)
        check.environment = environment{
                scope: d.file,
        }

        // Const and var declarations must not have initialization
        // cycles. We track them by remembering the current declaration
        // in check.decl. Initialization expressions depending on other
        // consts, vars, or functions, add dependencies to the current
        // check.decl.
        switch obj := obj.(type) {
        case *Const:
                check.decl = d // new package-level const decl
                check.constDecl(obj, d.vtyp, d.init, d.inherited)
        case *Var:
                check.decl = d // new package-level var decl
                check.varDecl(obj, d.lhs, d.vtyp, d.init)
        case *TypeName:
                // invalid recursive types are detected via path
                check.typeDecl(obj, d.tdecl, def)
                check.collectMethods(obj) // methods can only be added to top-level types
        case *Func:
                // functions may be recursive - no need to track dependencies
                check.funcDecl(obj, d)
        default:
                unreachable()
        }
}

// validCycle checks if the cycle starting with obj is valid and
// reports an error if it is not.
func (check *Checker) validCycle(obj Object) (valid bool) {
        // The object map contains the package scope objects and the non-interface methods.
        if debug {
                info := check.objMap[obj]
                inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
                isPkgObj := obj.Parent() == check.pkg.scope
                if isPkgObj != inObjMap {
                        check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
                        unreachable()
                }
        }

        // Count cycle objects.
        assert(obj.color() >= grey)
        start := obj.color() - grey // index of obj in objPath
        cycle := check.objPath[start:]
        tparCycle := false // if set, the cycle is through a type parameter list
        nval := 0          // number of (constant or variable) values in the cycle; valid if !generic
        ndef := 0          // number of type definitions in the cycle; valid if !generic
loop:
        for _, obj := range cycle {
                switch obj := obj.(type) {
                case *Const, *Var:
                        nval++
                case *TypeName:
                        // If we reach a generic type that is part of a cycle
                        // and we are in a type parameter list, we have a cycle
                        // through a type parameter list, which is invalid.
                        if check.inTParamList && isGeneric(obj.typ) {
                                tparCycle = true
                                break loop
                        }

                        // Determine if the type name is an alias or not. For
                        // package-level objects, use the object map which
                        // provides syntactic information (which doesn't rely
                        // on the order in which the objects are set up). For
                        // local objects, we can rely on the order, so use
                        // the object's predicate.
                        // TODO(gri) It would be less fragile to always access
                        // the syntactic information. We should consider storing
                        // this information explicitly in the object.
                        var alias bool
                        if check.conf._EnableAlias {
                                alias = obj.IsAlias()
                        } else {
                                if d := check.objMap[obj]; d != nil {
                                        alias = d.tdecl.Assign.IsValid() // package-level object
                                } else {
                                        alias = obj.IsAlias() // function local object
                                }
                        }
                        if !alias {
                                ndef++
                        }
                case *Func:
                        // ignored for now
                default:
                        unreachable()
                }
        }

        if check.conf._Trace {
                check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
                if tparCycle {
                        check.trace(obj.Pos(), "## cycle contains: generic type in a type parameter list")
                } else {
                        check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
                }
                defer func() {
                        if valid {
                                check.trace(obj.Pos(), "=> cycle is valid")
                        } else {
                                check.trace(obj.Pos(), "=> error: cycle is invalid")
                        }
                }()
        }

        if !tparCycle {
                // A cycle involving only constants and variables is invalid but we
                // ignore them here because they are reported via the initialization
                // cycle check.
                if nval == len(cycle) {
                        return true
                }

                // A cycle involving only types (and possibly functions) must have at least
                // one type definition to be permitted: If there is no type definition, we
                // have a sequence of alias type names which will expand ad infinitum.
                if nval == 0 && ndef > 0 {
                        return true
                }
        }

        check.cycleError(cycle)
        return false
}

// cycleError reports a declaration cycle starting with
// the object in cycle that is "first" in the source.
func (check *Checker) cycleError(cycle []Object) {
        // name returns the (possibly qualified) object name.
        // This is needed because with generic types, cycles
        // may refer to imported types. See go.dev/issue/50788.
        // TODO(gri) Thus functionality is used elsewhere. Factor it out.
        name := func(obj Object) string {
                return packagePrefix(obj.Pkg(), check.qualifier) + obj.Name()
        }

        // TODO(gri) Should we start with the last (rather than the first) object in the cycle
        //           since that is the earliest point in the source where we start seeing the
        //           cycle? That would be more consistent with other error messages.
        i := firstInSrc(cycle)
        obj := cycle[i]
        objName := name(obj)
        // If obj is a type alias, mark it as valid (not broken) in order to avoid follow-on errors.
        tname, _ := obj.(*TypeName)
        if tname != nil && tname.IsAlias() {
                // If we use Alias nodes, it is initialized with Typ[Invalid].
                // TODO(gri) Adjust this code if we initialize with nil.
                if !check.conf._EnableAlias {
                        check.validAlias(tname, Typ[Invalid])
                }
        }

        // report a more concise error for self references
        if len(cycle) == 1 {
                if tname != nil {
                        check.errorf(obj, InvalidDeclCycle, "invalid recursive type: %s refers to itself", objName)
                } else {
                        check.errorf(obj, InvalidDeclCycle, "invalid cycle in declaration: %s refers to itself", objName)
                }
                return
        }

        if tname != nil {
                check.errorf(obj, InvalidDeclCycle, "invalid recursive type %s", objName)
        } else {
                check.errorf(obj, InvalidDeclCycle, "invalid cycle in declaration of %s", objName)
        }
        for range cycle {
                check.errorf(obj, InvalidDeclCycle, "\t%s refers to", objName) // secondary error, \t indented
                i++
                if i >= len(cycle) {
                        i = 0
                }
                obj = cycle[i]
                objName = name(obj)
        }
        check.errorf(obj, InvalidDeclCycle, "\t%s", objName)
}

// firstInSrc reports the index of the object with the "smallest"
// source position in path. path must not be empty.
func firstInSrc(path []Object) int {
        fst, pos := 0, path[0].Pos()
        for i, t := range path[1:] {
                if cmpPos(t.Pos(), pos) < 0 {
                        fst, pos = i+1, t.Pos()
                }
        }
        return fst
}

type (
        decl interface {
                node() ast.Node
        }

        importDecl struct{ spec *ast.ImportSpec }
        constDecl  struct {
                spec      *ast.ValueSpec
                iota      int
                typ       ast.Expr
                init      []ast.Expr
                inherited bool
        }
        varDecl  struct{ spec *ast.ValueSpec }
        typeDecl struct{ spec *ast.TypeSpec }
        funcDecl struct{ decl *ast.FuncDecl }
)

func (d importDecl) node() ast.Node { return d.spec }
func (d constDecl) node() ast.Node  { return d.spec }
func (d varDecl) node() ast.Node    { return d.spec }
func (d typeDecl) node() ast.Node   { return d.spec }
func (d funcDecl) node() ast.Node   { return d.decl }

func (check *Checker) walkDecls(decls []ast.Decl, f func(decl)) {
        for _, d := range decls {
                check.walkDecl(d, f)
        }
}

func (check *Checker) walkDecl(d ast.Decl, f func(decl)) {
        switch d := d.(type) {
        case *ast.BadDecl:
                // ignore
        case *ast.GenDecl:
                var last *ast.ValueSpec // last ValueSpec with type or init exprs seen
                for iota, s := range d.Specs {
                        switch s := s.(type) {
                        case *ast.ImportSpec:
                                f(importDecl{s})
                        case *ast.ValueSpec:
                                switch d.Tok {
                                case token.CONST:
                                        // determine which initialization expressions to use
                                        inherited := true
                                        switch {
                                        case s.Type != nil || len(s.Values) > 0:
                                                last = s
                                                inherited = false
                                        case last == nil:
                                                last = new(ast.ValueSpec) // make sure last exists
                                                inherited = false
                                        }
                                        check.arityMatch(s, last)
                                        f(constDecl{spec: s, iota: iota, typ: last.Type, init: last.Values, inherited: inherited})
                                case token.VAR:
                                        check.arityMatch(s, nil)
                                        f(varDecl{s})
                                default:
                                        check.errorf(s, InvalidSyntaxTree, "invalid token %s", d.Tok)
                                }
                        case *ast.TypeSpec:
                                f(typeDecl{s})
                        default:
                                check.errorf(s, InvalidSyntaxTree, "unknown ast.Spec node %T", s)
                        }
                }
        case *ast.FuncDecl:
                f(funcDecl{d})
        default:
                check.errorf(d, InvalidSyntaxTree, "unknown ast.Decl node %T", d)
        }
}

func (check *Checker) constDecl(obj *Const, typ, init ast.Expr, inherited bool) {
        assert(obj.typ == nil)

        // use the correct value of iota
        defer func(iota constant.Value, errpos positioner) {
                check.iota = iota
                check.errpos = errpos
        }(check.iota, check.errpos)
        check.iota = obj.val
        check.errpos = nil

        // provide valid constant value under all circumstances
        obj.val = constant.MakeUnknown()

        // determine type, if any
        if typ != nil {
                t := check.typ(typ)
                if !isConstType(t) {
                        // don't report an error if the type is an invalid C (defined) type
                        // (go.dev/issue/22090)
                        if isValid(under(t)) {
                                check.errorf(typ, InvalidConstType, "invalid constant type %s", t)
                        }
                        obj.typ = Typ[Invalid]
                        return
                }
                obj.typ = t
        }

        // check initialization
        var x operand
        if init != nil {
                if inherited {
                        // The initialization expression is inherited from a previous
                        // constant declaration, and (error) positions refer to that
                        // expression and not the current constant declaration. Use
                        // the constant identifier position for any errors during
                        // init expression evaluation since that is all we have
                        // (see issues go.dev/issue/42991, go.dev/issue/42992).
                        check.errpos = atPos(obj.pos)
                }
                check.expr(nil, &x, init)
        }
        check.initConst(obj, &x)
}

func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
        assert(obj.typ == nil)

        // determine type, if any
        if typ != nil {
                obj.typ = check.varType(typ)
                // We cannot spread the type to all lhs variables if there
                // are more than one since that would mark them as checked
                // (see Checker.objDecl) and the assignment of init exprs,
                // if any, would not be checked.
                //
                // TODO(gri) If we have no init expr, we should distribute
                // a given type otherwise we need to re-evalate the type
                // expr for each lhs variable, leading to duplicate work.
        }

        // check initialization
        if init == nil {
                if typ == nil {
                        // error reported before by arityMatch
                        obj.typ = Typ[Invalid]
                }
                return
        }

        if lhs == nil || len(lhs) == 1 {
                assert(lhs == nil || lhs[0] == obj)
                var x operand
                check.expr(obj.typ, &x, init)
                check.initVar(obj, &x, "variable declaration")
                return
        }

        if debug {
                // obj must be one of lhs
                found := false
                for _, lhs := range lhs {
                        if obj == lhs {
                                found = true
                                break
                        }
                }
                if !found {
                        panic("inconsistent lhs")
                }
        }

        // We have multiple variables on the lhs and one init expr.
        // Make sure all variables have been given the same type if
        // one was specified, otherwise they assume the type of the
        // init expression values (was go.dev/issue/15755).
        if typ != nil {
                for _, lhs := range lhs {
                        lhs.typ = obj.typ
                }
        }

        check.initVars(lhs, []ast.Expr{init}, nil)
}

// isImportedConstraint reports whether typ is an imported type constraint.
func (check *Checker) isImportedConstraint(typ Type) bool {
        named := asNamed(typ)
        if named == nil || named.obj.pkg == check.pkg || named.obj.pkg == nil {
                return false
        }
        u, _ := named.under().(*Interface)
        return u != nil && !u.IsMethodSet()
}

func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName) {
        assert(obj.typ == nil)

        var rhs Type
        check.later(func() {
                if t := asNamed(obj.typ); t != nil { // type may be invalid
                        check.validType(t)
                }
                // If typ is local, an error was already reported where typ is specified/defined.
                _ = check.isImportedConstraint(rhs) && check.verifyVersionf(tdecl.Type, go1_18, "using type constraint %s", rhs)
        }).describef(obj, "validType(%s)", obj.Name())

        aliasDecl := tdecl.Assign.IsValid()
        if aliasDecl && tdecl.TypeParams.NumFields() != 0 {
                // The parser will ensure this but we may still get an invalid AST.
                // Complain and continue as regular type definition.
                check.error(atPos(tdecl.Assign), BadDecl, "generic type cannot be alias")
                aliasDecl = false
        }

        // alias declaration
        if aliasDecl {
                check.verifyVersionf(atPos(tdecl.Assign), go1_9, "type aliases")
                if check.conf._EnableAlias {
                        // TODO(gri) Should be able to use nil instead of Typ[Invalid] to mark
                        //           the alias as incomplete. Currently this causes problems
                        //           with certain cycles. Investigate.
                        alias := check.newAlias(obj, Typ[Invalid])
                        setDefType(def, alias)
                        rhs = check.definedType(tdecl.Type, obj)
                        assert(rhs != nil)
                        alias.fromRHS = rhs
                        _Unalias(alias) // resolve alias.actual
                } else {
                        check.brokenAlias(obj)
                        rhs = check.typ(tdecl.Type)
                        check.validAlias(obj, rhs)
                }
                return
        }

        // type definition or generic type declaration
        named := check.newNamed(obj, nil, nil)
        setDefType(def, named)

        if tdecl.TypeParams != nil {
                check.openScope(tdecl, "type parameters")
                defer check.closeScope()
                check.collectTypeParams(&named.tparams, tdecl.TypeParams)
        }

        // determine underlying type of named
        rhs = check.definedType(tdecl.Type, obj)
        assert(rhs != nil)
        named.fromRHS = rhs

        // If the underlying type was not set while type-checking the right-hand
        // side, it is invalid and an error should have been reported elsewhere.
        if named.underlying == nil {
                named.underlying = Typ[Invalid]
        }

        // Disallow a lone type parameter as the RHS of a type declaration (go.dev/issue/45639).
        // We don't need this restriction anymore if we make the underlying type of a type
        // parameter its constraint interface: if the RHS is a lone type parameter, we will
        // use its underlying type (like we do for any RHS in a type declaration), and its
        // underlying type is an interface and the type declaration is well defined.
        if isTypeParam(rhs) {
                check.error(tdecl.Type, MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
                named.underlying = Typ[Invalid]
        }
}

func (check *Checker) collectTypeParams(dst **TypeParamList, list *ast.FieldList) {
        var tparams []*TypeParam
        // Declare type parameters up-front, with empty interface as type bound.
        // The scope of type parameters starts at the beginning of the type parameter
        // list (so we can have mutually recursive parameterized interfaces).
        for _, f := range list.List {
                tparams = check.declareTypeParams(tparams, f.Names)
        }

        // Set the type parameters before collecting the type constraints because
        // the parameterized type may be used by the constraints (go.dev/issue/47887).
        // Example: type T[P T[P]] interface{}
        *dst = bindTParams(tparams)

        // Signal to cycle detection that we are in a type parameter list.
        // We can only be inside one type parameter list at any given time:
        // function closures may appear inside a type parameter list but they
        // cannot be generic, and their bodies are processed in delayed and
        // sequential fashion. Note that with each new declaration, we save
        // the existing environment and restore it when done; thus inTPList is
        // true exactly only when we are in a specific type parameter list.
        assert(!check.inTParamList)
        check.inTParamList = true
        defer func() {
                check.inTParamList = false
        }()

        index := 0
        for _, f := range list.List {
                var bound Type
                // NOTE: we may be able to assert that f.Type != nil here, but this is not
                // an invariant of the AST, so we are cautious.
                if f.Type != nil {
                        bound = check.bound(f.Type)
                        if isTypeParam(bound) {
                                // We may be able to allow this since it is now well-defined what
                                // the underlying type and thus type set of a type parameter is.
                                // But we may need some additional form of cycle detection within
                                // type parameter lists.
                                check.error(f.Type, MisplacedTypeParam, "cannot use a type parameter as constraint")
                                bound = Typ[Invalid]
                        }
                } else {
                        bound = Typ[Invalid]
                }
                for i := range f.Names {
                        tparams[index+i].bound = bound
                }
                index += len(f.Names)
        }
}

func (check *Checker) bound(x ast.Expr) Type {
        // A type set literal of the form ~T and A|B may only appear as constraint;
        // embed it in an implicit interface so that only interface type-checking
        // needs to take care of such type expressions.
        wrap := false
        switch op := x.(type) {
        case *ast.UnaryExpr:
                wrap = op.Op == token.TILDE
        case *ast.BinaryExpr:
                wrap = op.Op == token.OR
        }
        if wrap {
                x = &ast.InterfaceType{Methods: &ast.FieldList{List: []*ast.Field{{Type: x}}}}
                t := check.typ(x)
                // mark t as implicit interface if all went well
                if t, _ := t.(*Interface); t != nil {
                        t.implicit = true
                }
                return t
        }
        return check.typ(x)
}

func (check *Checker) declareTypeParams(tparams []*TypeParam, names []*ast.Ident) []*TypeParam {
        // Use Typ[Invalid] for the type constraint to ensure that a type
        // is present even if the actual constraint has not been assigned
        // yet.
        // TODO(gri) Need to systematically review all uses of type parameter
        //           constraints to make sure we don't rely on them if they
        //           are not properly set yet.
        for _, name := range names {
                tname := NewTypeName(name.Pos(), check.pkg, name.Name, nil)
                tpar := check.newTypeParam(tname, Typ[Invalid])          // assigns type to tpar as a side-effect
                check.declare(check.scope, name, tname, check.scope.pos) // TODO(gri) check scope position
                tparams = append(tparams, tpar)
        }

        if check.conf._Trace && len(names) > 0 {
                check.trace(names[0].Pos(), "type params = %v", tparams[len(tparams)-len(names):])
        }

        return tparams
}

func (check *Checker) collectMethods(obj *TypeName) {
        // get associated methods
        // (Checker.collectObjects only collects methods with non-blank names;
        // Checker.resolveBaseTypeName ensures that obj is not an alias name
        // if it has attached methods.)
        methods := check.methods[obj]
        if methods == nil {
                return
        }
        delete(check.methods, obj)
        assert(!check.objMap[obj].tdecl.Assign.IsValid()) // don't use TypeName.IsAlias (requires fully set up object)

        // use an objset to check for name conflicts
        var mset objset

        // spec: "If the base type is a struct type, the non-blank method
        // and field names must be distinct."
        base := asNamed(obj.typ) // shouldn't fail but be conservative
        if base != nil {
                assert(base.TypeArgs().Len() == 0) // collectMethods should not be called on an instantiated type

                // See go.dev/issue/52529: we must delay the expansion of underlying here, as
                // base may not be fully set-up.
                check.later(func() {
                        check.checkFieldUniqueness(base)
                }).describef(obj, "verifying field uniqueness for %v", base)

                // Checker.Files may be called multiple times; additional package files
                // may add methods to already type-checked types. Add pre-existing methods
                // so that we can detect redeclarations.
                for i := 0; i < base.NumMethods(); i++ {
                        m := base.Method(i)
                        assert(m.name != "_")
                        assert(mset.insert(m) == nil)
                }
        }

        // add valid methods
        for _, m := range methods {
                // spec: "For a base type, the non-blank names of methods bound
                // to it must be unique."
                assert(m.name != "_")
                if alt := mset.insert(m); alt != nil {
                        if alt.Pos().IsValid() {
                                check.errorf(m, DuplicateMethod, "method %s.%s already declared at %s", obj.Name(), m.name, alt.Pos())
                        } else {
                                check.errorf(m, DuplicateMethod, "method %s.%s already declared", obj.Name(), m.name)
                        }
                        continue
                }

                if base != nil {
                        base.AddMethod(m)
                }
        }
}

func (check *Checker) checkFieldUniqueness(base *Named) {
        if t, _ := base.under().(*Struct); t != nil {
                var mset objset
                for i := 0; i < base.NumMethods(); i++ {
                        m := base.Method(i)
                        assert(m.name != "_")
                        assert(mset.insert(m) == nil)
                }

                // Check that any non-blank field names of base are distinct from its
                // method names.
                for _, fld := range t.fields {
                        if fld.name != "_" {
                                if alt := mset.insert(fld); alt != nil {
                                        // Struct fields should already be unique, so we should only
                                        // encounter an alternate via collision with a method name.
                                        _ = alt.(*Func)

                                        // For historical consistency, we report the primary error on the
                                        // method, and the alt decl on the field.
                                        check.errorf(alt, DuplicateFieldAndMethod, "field and method with the same name %s", fld.name)
                                        check.reportAltDecl(fld)
                                }
                        }
                }
        }
}

func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
        assert(obj.typ == nil)

        // func declarations cannot use iota
        assert(check.iota == nil)

        sig := new(Signature)
        obj.typ = sig // guard against cycles

        // Avoid cycle error when referring to method while type-checking the signature.
        // This avoids a nuisance in the best case (non-parameterized receiver type) and
        // since the method is not a type, we get an error. If we have a parameterized
        // receiver type, instantiating the receiver type leads to the instantiation of
        // its methods, and we don't want a cycle error in that case.
        // TODO(gri) review if this is correct and/or whether we still need this?
        saved := obj.color_
        obj.color_ = black
        fdecl := decl.fdecl
        check.funcType(sig, fdecl.Recv, fdecl.Type)
        obj.color_ = saved

        if fdecl.Type.TypeParams.NumFields() > 0 && fdecl.Body == nil {
                check.softErrorf(fdecl.Name, BadDecl, "generic function is missing function body")
        }

        // function body must be type-checked after global declarations
        // (functions implemented elsewhere have no body)
        if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
                check.later(func() {
                        check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
                }).describef(obj, "func %s", obj.name)
        }
}

func (check *Checker) declStmt(d ast.Decl) {
        pkg := check.pkg

        check.walkDecl(d, func(d decl) {
                switch d := d.(type) {
                case constDecl:
                        top := len(check.delayed)

                        // declare all constants
                        lhs := make([]*Const, len(d.spec.Names))
                        for i, name := range d.spec.Names {
                                obj := NewConst(name.Pos(), pkg, name.Name, nil, constant.MakeInt64(int64(d.iota)))
                                lhs[i] = obj

                                var init ast.Expr
                                if i < len(d.init) {
                                        init = d.init[i]
                                }

                                check.constDecl(obj, d.typ, init, d.inherited)
                        }

                        // process function literals in init expressions before scope changes
                        check.processDelayed(top)

                        // spec: "The scope of a constant or variable identifier declared
                        // inside a function begins at the end of the ConstSpec or VarSpec
                        // (ShortVarDecl for short variable declarations) and ends at the
                        // end of the innermost containing block."
                        scopePos := d.spec.End()
                        for i, name := range d.spec.Names {
                                check.declare(check.scope, name, lhs[i], scopePos)
                        }

                case varDecl:
                        top := len(check.delayed)

                        lhs0 := make([]*Var, len(d.spec.Names))
                        for i, name := range d.spec.Names {
                                lhs0[i] = NewVar(name.Pos(), pkg, name.Name, nil)
                        }

                        // initialize all variables
                        for i, obj := range lhs0 {
                                var lhs []*Var
                                var init ast.Expr
                                switch len(d.spec.Values) {
                                case len(d.spec.Names):
                                        // lhs and rhs match
                                        init = d.spec.Values[i]
                                case 1:
                                        // rhs is expected to be a multi-valued expression
                                        lhs = lhs0
                                        init = d.spec.Values[0]
                                default:
                                        if i < len(d.spec.Values) {
                                                init = d.spec.Values[i]
                                        }
                                }
                                check.varDecl(obj, lhs, d.spec.Type, init)
                                if len(d.spec.Values) == 1 {
                                        // If we have a single lhs variable we are done either way.
                                        // If we have a single rhs expression, it must be a multi-
                                        // valued expression, in which case handling the first lhs
                                        // variable will cause all lhs variables to have a type
                                        // assigned, and we are done as well.
                                        if debug {
                                                for _, obj := range lhs0 {
                                                        assert(obj.typ != nil)
                                                }
                                        }
                                        break
                                }
                        }

                        // process function literals in init expressions before scope changes
                        check.processDelayed(top)

                        // declare all variables
                        // (only at this point are the variable scopes (parents) set)
                        scopePos := d.spec.End() // see constant declarations
                        for i, name := range d.spec.Names {
                                // see constant declarations
                                check.declare(check.scope, name, lhs0[i], scopePos)
                        }

                case typeDecl:
                        obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
                        // spec: "The scope of a type identifier declared inside a function
                        // begins at the identifier in the TypeSpec and ends at the end of
                        // the innermost containing block."
                        scopePos := d.spec.Name.Pos()
                        check.declare(check.scope, d.spec.Name, obj, scopePos)
                        // mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
                        obj.setColor(grey + color(check.push(obj)))
                        check.typeDecl(obj, d.spec, nil)
                        check.pop().setColor(black)
                default:
                        check.errorf(d.node(), InvalidSyntaxTree, "unknown ast.Decl node %T", d.node())
                }
        })
}