reflect: fully implement Type.AssignableTo

This fully implements AssignableTo, fixing a number of bugs in the
previous implementation. In particular, it now supports AssignableTo for
interface types.

src/reflect/set_test.go

@@ -196,34 +196,30 @@ func TestImplements(t *testing.T) {
 	}
 }
 
-// TODO: AssignableTo should be possible to implement now that Implements()
-// mostly works.
+var assignableTests = []struct {
+	x any
+	t any
+	b bool
+}{
+	{new(chan int), new(<-chan int), true},
+	{new(<-chan int), new(chan int), false},
+	{new(*int), new(IntPtr), true},
+	{new(IntPtr), new(*int), true},
+	{new(IntPtr), new(IntPtr1), false},
+	{new(Ch), new(<-chan any), true},
+	// test runs implementsTests too
+}
 
-//var assignableTests = []struct {
-//	x any
-//	t any
-//	b bool
-//}{
-//	{new(chan int), new(<-chan int), true},
-//	{new(<-chan int), new(chan int), false},
-//	{new(*int), new(IntPtr), true},
-//	{new(IntPtr), new(*int), true},
-//	{new(IntPtr), new(IntPtr1), false},
-//	{new(Ch), new(<-chan any), true},
-//	// test runs implementsTests too
-//}
-//
-//type IntPtr *int
-//type IntPtr1 *int
-//type Ch <-chan any
-//
-//func TestAssignableTo(t *testing.T) {
-//	for _, tt := range append(assignableTests, implementsTests...) {
-//		xv := TypeOf(tt.x).Elem()
-//		xt := TypeOf(tt.t).Elem()
-//		if b := xv.AssignableTo(xt); b != tt.b {
-//			t.Errorf("(%s).AssignableTo(%s) = %v, want %v", xv.String(), xt.String(), b, tt.b)
-//		}
-//	}
-//}
-//
+type IntPtr *int
+type IntPtr1 *int
+type Ch <-chan any
+
+func TestAssignableTo(t *testing.T) {
+	for _, tt := range append(assignableTests, implementsTests...) {
+		xv := TypeOf(tt.x).Elem()
+		xt := TypeOf(tt.t).Elem()
+		if b := xv.AssignableTo(xt); b != tt.b {
+			t.Errorf("(%s).AssignableTo(%s) = %v, want %v", xv.String(), xt.String(), b, tt.b)
+		}
+	}
+}

src/reflect/type.go

@@ -972,17 +972,52 @@ func (t *rawType) FieldAlign() int {
 // AssignableTo returns whether a value of type t can be assigned to a variable
 // of type u.
 func (t *rawType) AssignableTo(u Type) bool {
-	if t == u.(*rawType) {
-		return true
-	}
-
-	if u.Kind() == Interface && u.NumMethod() == 0 {
+	// Quotes come from the language spec:
+	// https://go.dev/ref/spec#Assignability
+	// > A value x of type V is assignable to a variable of type T ("x is
+	// > assignable to T") if one of the following conditions applies:
+	//   [...]
+	// (Replace T with t, and x with u).
+	u_raw := u.(*rawType)
+	if t == u_raw {
+		// > V and T are identical.
 		return true
 	}
 
 	if u.Kind() == Interface {
-		panic("reflect: unimplemented: AssignableTo with interface")
+		// > T is an interface type, but not a type parameter, and x implements
+		// > T.
+		u_itf := (*interfaceType)(unsafe.Pointer(u_raw.underlying()))
+		res := typeImplementsMethodSet(unsafe.Pointer(t), unsafe.Pointer(&u_itf.methods))
+		return res
 	}
+
+	t_named := t.isNamed()
+	u_named := u_raw.isNamed()
+	if t_named && u_named {
+		return false
+	}
+	if t.underlying() == u_raw.underlying() {
+		// > V and T have identical underlying types but are not type parameters
+		// > and at least one of V or T is not a named type.
+		return true
+	}
+
+	if t.Kind() == Chan && u_raw.Kind() == Chan {
+		// > V and T are channel types with identical element types, V is a
+		// > bidirectional channel, and at least one of V or T is not a named
+		// > type.
+		t_chan := (*elemType)(unsafe.Pointer(t.underlying()))
+		u_chan := (*elemType)(unsafe.Pointer(u_raw.underlying()))
+		if t_chan.elem != u_chan.elem {
+			return false
+		}
+		if t_chan.ChanDir() != BothDir {
+			return false
+		}
+		return true
+	}
+
 	return false
 }
 

src/reflect/value_test.go

@@ -592,15 +592,6 @@ func TestTinyNumMethods(t *testing.T) {
 	}
 }
 
-func TestAssignableTo(t *testing.T) {
-	var a any
-	refa := ValueOf(&a).Elem()
-	refa.Set(ValueOf(4))
-	if got, want := refa.Interface().(int), 4; got != want {
-		t.Errorf("AssignableTo / Set failed, got %v, want %v", got, want)
-	}
-}
-
 func TestConvert(t *testing.T) {
 	v := ValueOf(int64(3))
 	c := v.Convert(TypeOf(byte(0)))