Getting DeepEqual to work with maps whose key is an interface (#4360)

reflect: Getting DeepEqual to work with maps whose key is an interface

src/reflect/all_test.go

@@ -1102,6 +1102,20 @@ func init() {
 	cycleMap3["different"] = cycleMap3
 }
 
+type deepEqualInterface interface {
+	Foo()
+}
+type deepEqualConcrete struct {
+	int
+}
+
+func (deepEqualConcrete) Foo() {}
+
+var (
+	deepEqualConcrete1 = deepEqualConcrete{1}
+	deepEqualConcrete2 = deepEqualConcrete{2}
+)
+
 var deepEqualTests = []DeepEqualTest{
 	// Equalities
 	{nil, nil, true},
@@ -1119,6 +1133,7 @@ var deepEqualTests = []DeepEqualTest{
 	{[]byte{1, 2, 3}, []byte{1, 2, 3}, true},
 	{[]MyByte{1, 2, 3}, []MyByte{1, 2, 3}, true},
 	{MyBytes{1, 2, 3}, MyBytes{1, 2, 3}, true},
+	{map[deepEqualInterface]string{deepEqualConcrete1: "a"}, map[deepEqualInterface]string{deepEqualConcrete1: "a"}, true},
 
 	// Inequalities
 	{1, 2, false},
@@ -1140,6 +1155,7 @@ var deepEqualTests = []DeepEqualTest{
 	{fn3, fn3, false},
 	{[][]int{{1}}, [][]int{{2}}, false},
 	{&structWithSelfPtr{p: &structWithSelfPtr{s: "a"}}, &structWithSelfPtr{p: &structWithSelfPtr{s: "b"}}, false},
+	{map[deepEqualInterface]string{deepEqualConcrete1: "a"}, map[deepEqualInterface]string{deepEqualConcrete2: "a"}, false},
 
 	// Fun with floating point.
 	{math.NaN(), math.NaN(), false},

src/reflect/value.go

@@ -982,7 +982,9 @@ func (v Value) MapIndex(key Value) Value {
 	vkey := v.typecode.key()
 
 	// compare key type with actual key type of map
-	if !key.typecode.AssignableTo(vkey) {
+	// AssignableTo is not implemented for interfaces
+	// avoid: "reflect: unimplemented: AssignableTo with interface"
+	if vkey.Kind() != Interface && !key.typecode.AssignableTo(vkey) {
 		// type error?
 		panic("reflect.Value.MapIndex: incompatible types for key")
 	}