reflect: add MapKeys()

src/reflect/value.go

@@ -641,7 +641,27 @@ func (v Value) OverflowFloat(x float64) bool {
 }
 
 func (v Value) MapKeys() []Value {
-	panic("unimplemented: (reflect.Value).MapKeys()")
+	if v.Kind() != Map {
+		panic(&ValueError{Method: "MapKeys", Kind: v.Kind()})
+	}
+
+	// empty map
+	if v.Len() == 0 {
+		return nil
+	}
+
+	keys := make([]Value, 0, v.Len())
+
+	it := hashmapNewIterator()
+	k := New(v.typecode.Key())
+	e := New(v.typecode.Elem())
+
+	for hashmapNext(v.pointer(), it, k.value, e.value) {
+		keys = append(keys, k.Elem())
+		k = New(v.typecode.Key())
+	}
+
+	return keys
 }
 
 //go:linkname hashmapStringGet runtime.hashmapStringGetUnsafePointer

src/reflect/value_test.go

@@ -86,6 +86,17 @@ func TestMap(t *testing.T) {
 		t.Errorf("MapRange return unexpected keys: got %v, want %v", gotKeys, wantKeys)
 	}
 
+	refMapKeys := mref.MapKeys()
+	gotKeys = gotKeys[:0]
+	for _, v := range refMapKeys {
+		gotKeys = append(gotKeys, v.Interface().(string))
+	}
+
+	sort.Strings(gotKeys)
+	if !equal(gotKeys, wantKeys) {
+		t.Errorf("MapKeys return unexpected keys: got %v, want %v", gotKeys, wantKeys)
+	}
+
 	mref.SetMapIndex(ValueOf("bar"), Value{})
 	if _, ok := m["bar"]; ok {
 		t.Errorf("SetMapIndex failed to delete `bar`")