go/types: add Func.Signature method

Unfortunately we can't enforce the repr invariant
that Func.typ != nil without thinking about the
object color invariants. For now, return a trivial
Signature if typ == nil, which should never happen
in bug-free client code.

Fixes #65772

Change-Id: I7f89c6d8fdc8dcd4b8880572e54bb0ed9b6136eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/565375
Commit-Queue: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

api/next/65772.txt

@@ -0,0 +1 @@
+pkg go/types, method (*Func) Signature() *Signature #65772

doc/next/6-stdlib/99-minor/go/types/65772.md

@@ -0,0 +1,4 @@
+The [`Func`](/go/types#Func) type, which represents a function or
+method symbol, now has a [`Signature`](/go/types#Func.Signature)
+method that returns the function's type, which is always a
+`Signature`.

src/cmd/compile/internal/types2/object.go

@@ -374,14 +374,34 @@ type Func struct {
 // NewFunc returns a new function with the given signature, representing
 // the function's type.
 func NewFunc(pos syntax.Pos, pkg *Package, name string, sig *Signature) *Func {
- // don't store a (typed) nil signature
  var typ Type
  if sig != nil {
  typ = sig
+ } else {
+ // Don't store a (typed) nil *Signature.
+ // We can't simply replace it with new(Signature) either,
+ // as this would violate object.{Type,color} invariants.
+ // TODO(adonovan): propose to disallow NewFunc with nil *Signature.
  }
  return &Func{object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}, false, nil}
 }
 
+// Signature returns the signature (type) of the function or method.
+func (obj *Func) Signature() *Signature {
+ if obj.typ != nil {
+ return obj.typ.(*Signature) // normal case
+ }
+ // No signature: Signature was called either:
+ // - within go/types, before a FuncDecl's initially
+ // nil Func.Type was lazily populated, indicating
+ // a types bug; or
+ // - by a client after NewFunc(..., nil),
+ // which is arguably a client bug, but we need a
+ // proposal to tighten NewFunc's precondition.
+ // For now, return a trivial signature.
+ return new(Signature)
+}
+
 // FullName returns the package- or receiver-type-qualified name of
 // function or method obj.
 func (obj *Func) FullName() string {

src/cmd/compile/internal/types2/subst.go

@@ -421,7 +421,7 @@ func (subst *subster) termlist(in []*Term) (out []*Term, copied bool) {
 func replaceRecvType(in []*Func, old, new Type) (out []*Func, copied bool) {
  out = in
  for i, method := range in {
- sig := method.Type().(*Signature)
+ sig := method.Signature()
  if sig.recv != nil && sig.recv.Type() == old {
  if !copied {
  // Allocate a new methods slice before mutating for the first time.

src/go/types/api_test.go

@@ -2324,7 +2324,7 @@ func f(x int) { y := x; print(y) }
  wantParent = false
  }
  case *Func:
- if obj.Type().(*Signature).Recv() != nil { // method
+ if obj.Signature().Recv() != nil { // method
  wantParent = false
  }
  }
@@ -2615,9 +2615,9 @@ func fn() {
 
  // Methods and method fields
  {"concreteMethod", lookup("t").(*Named).Method(0)},
- {"recv", lookup("t").(*Named).Method(0).Type().(*Signature).Recv()},
- {"mParam", lookup("t").(*Named).Method(0).Type().(*Signature).Params().At(0)},
- {"mResult", lookup("t").(*Named).Method(0).Type().(*Signature).Results().At(0)},
+ {"recv", lookup("t").(*Named).Method(0).Signature().Recv()},
+ {"mParam", lookup("t").(*Named).Method(0).Signature().Params().At(0)},
+ {"mResult", lookup("t").(*Named).Method(0).Signature().Results().At(0)},
 
  // Interface methods
  {"interfaceMethod", lookup("i").Underlying().(*Interface).Method(0)},

src/go/types/issues_test.go

@@ -115,7 +115,7 @@ type T struct{} // receiver type after method declaration
  }
 
  m := f.Decls[0].(*ast.FuncDecl)
- res1 := defs[m.Name].(*Func).Type().(*Signature).Results().At(0)
+ res1 := defs[m.Name].(*Func).Signature().Results().At(0)
  res2 := defs[m.Type.Results.List[0].Names[0]].(*Var)
 
  if res1 != res2 {
@@ -369,7 +369,7 @@ func TestIssue28005(t *testing.T) {
  // must match the method's name per the choice in the source file.
  for i := 0; i < iface.NumMethods(); i++ {
  m := iface.Method(i)
- recvName := m.Type().(*Signature).Recv().Type().(*Named).Obj().Name()
+ recvName := m.Signature().Recv().Type().(*Named).Obj().Name()
  if recvName != m.Name() {
  t.Errorf("perm %v: got recv %s; want %s", perm, recvName, m.Name())
  }

src/go/types/resolver.go

@@ -395,8 +395,8 @@ func (check *Checker) collectObjects() {
  check.declarePkgObj(d.spec.Name, obj, &declInfo{file: fileScope, tdecl: d.spec})
  case funcDecl:
  name := d.decl.Name.Name
- obj := NewFunc(d.decl.Name.Pos(), pkg, name, nil)
- hasTParamError := false // avoid duplicate type parameter errors
+ obj := NewFunc(d.decl.Name.Pos(), pkg, name, nil) // signature set later
+ hasTParamError := false  // avoid duplicate type parameter errors
  if d.decl.Recv.NumFields() == 0 {
  // regular function
  if d.decl.Recv != nil {