reflect: convert map elements to an interface, if needed

src/reflect/value.go

@@ -1370,6 +1370,15 @@ func (v Value) SetMapIndex(key, elem Value) {
 		panic("reflect.Value.SetMapIndex: incompatible types for value")
 	}
 
+	// make elem an interface if it needs to be converted
+	if v.typecode.elem().Kind() == Interface && elem.typecode.Kind() != Interface {
+		intf := composeInterface(unsafe.Pointer(elem.typecode), elem.value)
+		elem = Value{
+			typecode: v.typecode.elem(),
+			value:    unsafe.Pointer(&intf),
+		}
+	}
+
 	if key.Kind() == String {
 		if del {
 			hashmapStringDelete(v.pointer(), *(*string)(key.value))

src/reflect/value_test.go

@@ -192,6 +192,24 @@ func TestTinyMap(t *testing.T) {
 	}
 }
 
+func TestMapInterfaceElem(t *testing.T) {
+	m := make(map[string]interface{})
+	refm := ValueOf(m)
+
+	four := ValueOf(4)
+	hello := ValueOf("hello")
+
+	refm.SetMapIndex(hello, four)
+
+	if v := refm.MapIndex(hello).Interface().(int); v != 4 {
+		t.Errorf("failed to get value assigned to interface via MapIndex")
+	}
+
+	if v := m["hello"].(int); v != 4 {
+		t.Errorf("failed to get value assigned to interface via direct lookup")
+	}
+}
+
 func TestTinySlice(t *testing.T) {
 	s := []int{0, 10, 20}
 	refs := ValueOf(s)