cmd/compile: fix PtrTo(t) for unnamed t with embedded fields

Fixes #8427.

Change-Id: I826a3bc4519845ad30d6dbaf058fe7ed7bee8db0
Reviewed-on: https://go-review.googlesource.com/12233
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -760,10 +760,11 @@ func dcommontype(s *Sym, ot int, t *Type) int {
 	}
 
 	var sptr *Sym
-	if t.Sym != nil && !Isptr[t.Etype] {
-		sptr = dtypesym(Ptrto(t))
+	tptr := Ptrto(t)
+	if !Isptr[t.Etype] && (t.Sym != nil || methods(tptr) != nil) {
+		sptr = dtypesym(tptr)
 	} else {
-		sptr = weaktypesym(Ptrto(t))
+		sptr = weaktypesym(tptr)
 	}
 
 	// All (non-reflect-allocated) Types share the same zero object.

src/reflect/all_test.go

@@ -4721,3 +4721,16 @@ func TestTypeOfTypeOf(t *testing.T) {
 	check("PtrTo", PtrTo(TypeOf(T{})))
 	check("SliceOf", SliceOf(TypeOf(T{})))
 }
+
+type XM struct{}
+
+func (*XM) String() string { return "" }
+
+func TestPtrToMethods(t *testing.T) {
+	var y struct{ XM }
+	yp := New(TypeOf(y)).Interface()
+	_, ok := yp.(fmt.Stringer)
+	if !ok {
+		t.Fatal("does not implement Stringer, but should")
+	}
+}