reflect: fix iterating over maps with interface{} keys

Fixes #3794

src/reflect/value.go

@@ -914,10 +914,11 @@ func (v Value) MapKeys() []Value {
 	e := New(v.typecode.Elem())
 
 	keyType := v.typecode.key()
-	isKeyStoredAsInterface := keyType.Kind() != String && !keyType.isBinary()
+	keyTypeIsEmptyInterface := keyType.Kind() == Interface && keyType.NumMethod() == 0
+	shouldUnpackInterface := !keyTypeIsEmptyInterface && keyType.Kind() != String && !keyType.isBinary()
 
 	for hashmapNext(v.pointer(), it, k.value, e.value) {
-		if isKeyStoredAsInterface {
+		if shouldUnpackInterface {
 			intf := *(*interface{})(k.value)
 			v := ValueOf(intf)
 			keys = append(keys, v)
@@ -992,12 +993,14 @@ func (v Value) MapRange() *MapIter {
 	}
 
 	keyType := v.typecode.key()
-	isKeyStoredAsInterface := keyType.Kind() != String && !keyType.isBinary()
+
+	keyTypeIsEmptyInterface := keyType.Kind() == Interface && keyType.NumMethod() == 0
+	shouldUnpackInterface := !keyTypeIsEmptyInterface && keyType.Kind() != String && !keyType.isBinary()
 
 	return &MapIter{
-		m:            v,
+		m:                  v,
-		it:           hashmapNewIterator(),
+		it:                 hashmapNewIterator(),
-		keyInterface: isKeyStoredAsInterface,
+		unpackKeyInterface: shouldUnpackInterface,
 	}
 }
 
@@ -1007,8 +1010,8 @@ type MapIter struct {
 	key Value
 	val Value
 
-	valid        bool
+	valid              bool
-	keyInterface bool
+	unpackKeyInterface bool
 }
 
 func (it *MapIter) Key() Value {
@@ -1016,7 +1019,7 @@ func (it *MapIter) Key() Value {
 		panic("reflect.MapIter.Key called on invalid iterator")
 	}
 
-	if it.keyInterface {
+	if it.unpackKeyInterface {
 		intf := *(*interface{})(it.key.value)
 		v := ValueOf(intf)
 		return v

src/reflect/value_test.go

@@ -191,6 +191,24 @@ func TestTinyMap(t *testing.T) {
 	if _, ok := utIterKey.Interface().(unmarshalerText); !ok {
 		t.Errorf("Map keys via MapIter() have wrong type: %v", utIterKey.Type().String())
 	}
+
+	{
+		m := map[any]any{1: 2}
+		rv := ValueOf(m)
+		iter := rv.MapRange()
+
+		iter.Next()
+		k := iter.Key()
+		if k.Kind().String() != "interface" {
+			t.Errorf("map[any]any MapRange has wrong key type: %v", k.Kind().String())
+		}
+
+		keys := rv.MapKeys()
+		if k := keys[0]; k.Kind().String() != "interface" {
+			t.Errorf("map[any]any MapRange has wrong key type: %v", k.Kind().String())
+		}
+
+	}
 }
 
 // For an interface type, it returns the number of exported and unexported methods.