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// Copyright 2021 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 types2
import (
// ----------------------------------------------------------------------------
// API
// A Signature represents a (non-builtin) function or method type.
// The receiver is ignored when comparing signatures for identity.
type Signature struct {
// We need to keep the scope in Signature (rather than passing it around
// and store it in the Func Object) because when type-checking a function
// literal we call the general type checker which returns a general Type.
// We then unpack the *Signature and use the scope for the literal body.
rparams *TParamList // receiver type parameters from left to right, or nil
tparams *TParamList // type parameters from left to right, or nil
scope *Scope // function scope, present for package-local signatures
recv *Var // nil if not a method
params *Tuple // (incoming) parameters from left to right; or nil
results *Tuple // (outgoing) results from left to right; or nil
variadic bool // true if the last parameter's type is of the form ...T (or string, for append built-in only)
// NewSignature returns a new function type for the given receiver, parameters,
// and results, either of which may be nil. If variadic is set, the function
// is variadic, it must have at least one parameter, and the last parameter
// must be of unnamed slice type.
func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
if variadic {
n := params.Len()
if n == 0 {
panic("variadic function must have at least one parameter")
if _, ok := params.At(n - 1).typ.(*Slice); !ok {
panic("variadic parameter must be of unnamed slice type")
return &Signature{recv: recv, params: params, results: results, variadic: variadic}
// Recv returns the receiver of signature s (if a method), or nil if a
// function. It is ignored when comparing signatures for identity.
// For an abstract method, Recv returns the enclosing interface either
// as a *Named or an *Interface. Due to embedding, an interface may
// contain methods whose receiver type is a different interface.
func (s *Signature) Recv() *Var { return s.recv }
// TParams returns the type parameters of signature s, or nil.
func (s *Signature) TParams() *TParamList { return s.tparams }
// SetTParams sets the type parameters of signature s.
func (s *Signature) SetTParams(tparams []*TypeParam) { s.tparams = bindTParams(tparams) }
// RParams returns the receiver type parameters of signature s, or nil.
func (s *Signature) RParams() *TParamList { return s.rparams }
// SetRParams sets the receiver type params of signature s.
func (s *Signature) SetRParams(rparams []*TypeParam) { s.rparams = bindTParams(rparams) }
// Params returns the parameters of signature s, or nil.
func (s *Signature) Params() *Tuple { return s.params }
// Results returns the results of signature s, or nil.
func (s *Signature) Results() *Tuple { return s.results }
// Variadic reports whether the signature s is variadic.
func (s *Signature) Variadic() bool { return s.variadic }
func (s *Signature) Underlying() Type { return s }
func (s *Signature) String() string { return TypeString(s, nil) }
// ----------------------------------------------------------------------------
// Implementation
// Disabled by default, but enabled when running tests (via types_test.go).
var acceptMethodTypeParams bool
// funcType type-checks a function or method type.
func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []*syntax.Field, ftyp *syntax.FuncType) {
check.openScope(ftyp, "function")
check.scope.isFunc = true
check.recordScope(ftyp, check.scope)
sig.scope = check.scope
defer check.closeScope()
var recvTyp syntax.Expr // rewritten receiver type; valid if != nil
if recvPar != nil {
// collect generic receiver type parameters, if any
// - a receiver type parameter is like any other type parameter, except that it is declared implicitly
// - the receiver specification acts as local declaration for its type parameters, which may be blank
_, rname, rparams := check.unpackRecv(recvPar.Type, true)
if len(rparams) > 0 {
// Blank identifiers don't get declared and regular type-checking of the instantiated
// parameterized receiver type expression fails in Checker.collectParams of receiver.
// Identify blank type parameters and substitute each with a unique new identifier named
// "n_" (where n is the parameter index) and which cannot conflict with any user-defined
// name.
var smap map[*syntax.Name]*syntax.Name // substitution map from "_" to "!n" identifiers
for i, p := range rparams {
if p.Value == "_" {
new := *p
new.Value = fmt.Sprintf("%d_", i)
rparams[i] = &new // use n_ identifier instead of _ so it can be looked up
if smap == nil {
smap = make(map[*syntax.Name]*syntax.Name)
smap[p] = &new
if smap != nil {
// blank identifiers were found => use rewritten receiver type
recvTyp = isubst(recvPar.Type, smap)
rlist := make([]*TypeParam, len(rparams))
for i, rparam := range rparams {
rlist[i] = check.declareTypeParam(rparam)
sig.rparams = bindTParams(rlist)
// determine receiver type to get its type parameters
// and the respective type parameter bounds
var recvTParams []*TypeParam
if rname != nil {
// recv should be a Named type (otherwise an error is reported elsewhere)
// Also: Don't report an error via genericType since it will be reported
// again when we type-check the signature.
// TODO(gri) maybe the receiver should be marked as invalid instead?
if recv := asNamed(check.genericType(rname, false)); recv != nil {
recvTParams = recv.TParams().list()
// provide type parameter bounds
// - only do this if we have the right number (otherwise an error is reported elsewhere)
if sig.RParams().Len() == len(recvTParams) {
// We have a list of *TypeNames but we need a list of Types.
list := make([]Type, sig.RParams().Len())
for i, t := range sig.RParams().list() {
list[i] = t
smap := makeSubstMap(recvTParams, list)
for i, tpar := range sig.RParams().list() {
bound := recvTParams[i].bound
// bound is (possibly) parameterized in the context of the
// receiver type declaration. Substitute parameters for the
// current context.
tpar.bound = check.subst(tpar.obj.pos, bound, smap, nil)
if tparams != nil {
sig.tparams = check.collectTypeParams(tparams)
// Always type-check method type parameters but complain if they are not enabled.
// (A separate check is needed when type-checking interface method signatures because
// they don't have a receiver specification.)
if recvPar != nil && !acceptMethodTypeParams {
check.error(ftyp, "methods cannot have type parameters")
// Value (non-type) parameters' scope starts in the function body. Use a temporary scope for their
// declarations and then squash that scope into the parent scope (and report any redeclarations at
// that time).
scope := NewScope(check.scope, nopos, nopos, "function body (temp. scope)")
var recvList []*Var // TODO(gri) remove the need for making a list here
if recvPar != nil {
recvList, _ = check.collectParams(scope, []*syntax.Field{recvPar}, recvTyp, false) // use rewritten receiver type, if any
params, variadic := check.collectParams(scope, ftyp.ParamList, nil, true)
results, _ := check.collectParams(scope, ftyp.ResultList, nil, false)
scope.Squash(func(obj, alt Object) {
var err error_
err.errorf(obj, "%s redeclared in this block", obj.Name())
if recvPar != nil {
// recv parameter list present (may be empty)
// spec: "The receiver is specified via an extra parameter section preceding the
// method name. That parameter section must declare a single parameter, the receiver."
var recv *Var
switch len(recvList) {
case 0:
// error reported by resolver
recv = NewParam(nopos, nil, "", Typ[Invalid]) // ignore recv below
// more than one receiver
check.error(recvList[len(recvList)-1].Pos(), "method must have exactly one receiver")
fallthrough // continue with first receiver
case 1:
recv = recvList[0]
// TODO(gri) We should delay rtyp expansion to when we actually need the
// receiver; thus all checks here should be delayed to later.
rtyp, _ := deref(recv.typ)
// spec: "The receiver type must be of the form T or *T where T is a type name."
// (ignore invalid types - error was reported before)
if rtyp != Typ[Invalid] {
var err string
switch T := rtyp.(type) {
case *Named:
// spec: "The type denoted by T is called the receiver base type; it must not
// be a pointer or interface type and it must be declared in the same package
// as the method."
if T.obj.pkg != check.pkg {
err = "type not defined in this package"
if check.conf.CompilerErrorMessages {
check.errorf(recv.pos, "cannot define new methods on non-local type %s", recv.typ)
err = ""
} else {
// The underlying type of a receiver base type can be a type parameter;
// e.g. for methods with a generic receiver T[P] with type T[P any] P.
underIs(T, func(u Type) bool {
switch u := u.(type) {
case *Basic:
// unsafe.Pointer is treated like a regular pointer
if u.kind == UnsafePointer {
err = "unsafe.Pointer"
return false
case *Pointer, *Interface:
err = "pointer or interface type"
return false
return true
case *Basic:
err = "basic or unnamed type"
if check.conf.CompilerErrorMessages {
check.errorf(recv.pos, "cannot define new methods on non-local type %s", recv.typ)
err = ""
check.errorf(recv.pos, "invalid receiver type %s", recv.typ)
if err != "" {
check.errorf(recv.pos, "invalid receiver type %s (%s)", recv.typ, err)
// ok to continue
sig.recv = recv
sig.params = NewTuple(params...)
sig.results = NewTuple(results...)
sig.variadic = variadic
// collectParams declares the parameters of list in scope and returns the corresponding
// variable list. If type0 != nil, it is used instead of the first type in list.
func (check *Checker) collectParams(scope *Scope, list []*syntax.Field, type0 syntax.Expr, variadicOk bool) (params []*Var, variadic bool) {
if list == nil {
var named, anonymous bool
var typ Type
var prev syntax.Expr
for i, field := range list {
ftype := field.Type
// type-check type of grouped fields only once
if ftype != prev {
prev = ftype
if i == 0 && type0 != nil {
ftype = type0
if t, _ := ftype.(*syntax.DotsType); t != nil {
ftype = t.Elem
if variadicOk && i == len(list)-1 {
variadic = true
} else {
check.softErrorf(t, "can only use ... with final parameter in list")
// ignore ... and continue
typ = check.varType(ftype)
// The parser ensures that f.Tag is nil and we don't
// care if a constructed AST contains a non-nil tag.
if field.Name != nil {
// named parameter
name := field.Name.Value
if name == "" {
check.error(field.Name, invalidAST+"anonymous parameter")
// ok to continue
par := NewParam(field.Name.Pos(), check.pkg, name, typ)
check.declare(scope, field.Name, par, scope.pos)
params = append(params, par)
named = true
} else {
// anonymous parameter
par := NewParam(field.Pos(), check.pkg, "", typ)
check.recordImplicit(field, par)
params = append(params, par)
anonymous = true
if named && anonymous {
check.error(list[0], invalidAST+"list contains both named and anonymous parameters")
// ok to continue
// For a variadic function, change the last parameter's type from T to []T.
// Since we type-checked T rather than ...T, we also need to retro-actively
// record the type for ...T.
if variadic {
last := params[len(params)-1]
last.typ = &Slice{elem: last.typ}
check.recordTypeAndValue(list[len(list)-1].Type, typexpr, last.typ, nil)
// isubst returns an x with identifiers substituted per the substitution map smap.
// isubst only handles the case of (valid) method receiver type expressions correctly.
func isubst(x syntax.Expr, smap map[*syntax.Name]*syntax.Name) syntax.Expr {
switch n := x.(type) {
case *syntax.Name:
if alt := smap[n]; alt != nil {
return alt
// case *syntax.StarExpr:
// X := isubst(n.X, smap)
// if X != n.X {
// new := *n
// new.X = X
// return &new
// }
case *syntax.Operation:
if n.Op == syntax.Mul && n.Y == nil {
X := isubst(n.X, smap)
if X != n.X {
new := *n
new.X = X
return &new
case *syntax.IndexExpr:
Index := isubst(n.Index, smap)
if Index != n.Index {
new := *n
new.Index = Index
return &new
case *syntax.ListExpr:
var elems []syntax.Expr
for i, elem := range n.ElemList {
new := isubst(elem, smap)
if new != elem {
if elems == nil {
elems = make([]syntax.Expr, len(n.ElemList))
copy(elems, n.ElemList)
elems[i] = new
if elems != nil {
new := *n
new.ElemList = elems
return &new
case *syntax.ParenExpr:
return isubst(n.X, smap) // no need to keep parentheses
// Other receiver type expressions are invalid.
// It's fine to ignore those here as they will
// be checked elsewhere.
return x