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 golang/go#65772

Change-Id: I7f89c6d8fdc8dcd4b8880572e54bb0ed9b6136eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/565375
Commit-Queue: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

9 files changed, 57 insertions, 12 deletions

diff --git a/api/next/65772.txt b/api/next/65772.txt
new file mode 100644
index 0000000000..1244df8dd3
--- /dev/null
+++ b/api/next/65772.txt
@@ -0,0 +1 @@
+pkg go/types, method (*Func) Signature() *Signature #65772
diff --git a/doc/next/6-stdlib/99-minor/go/types/65772.md b/doc/next/6-stdlib/99-minor/go/types/65772.md
new file mode 100644
index 0000000000..33e949db3d
--- /dev/null
+++ b/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`.
\ No newline at end of file
diff --git a/src/cmd/compile/internal/types2/object.go b/src/cmd/compile/internal/types2/object.go
index 776986f111..3026777cad 100644
--- a/src/cmd/compile/internal/types2/object.go
+++ b/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 {
diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index fa636a1e1e..215d1f2d4f 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/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.
diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 5bc4e8a61f..564bbc2423 100644
--- a/src/go/types/api_test.go
+++ b/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)},
diff --git a/src/go/types/issues_test.go b/src/go/types/issues_test.go
index 4f4bf6f077..379d833bf2 100644
--- a/src/go/types/issues_test.go
+++ b/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())
 			}
diff --git a/src/go/types/object.go b/src/go/types/object.go
index 400b47ebe1..d564d37e7a 100644
--- a/src/go/types/object.go
+++ b/src/go/types/object.go
@@ -377,14 +377,34 @@ type Func struct {
 // NewFunc returns a new function with the given signature, representing
 // the function's type.
 func NewFunc(pos token.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 {
diff --git a/src/go/types/resolver.go b/src/go/types/resolver.go
index f336057c53..918e18de3e 100644
--- a/src/go/types/resolver.go
+++ b/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 {
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index 1d180abb65..a3ea16d9b9 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -424,7 +424,7 @@ 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.