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

This reverts commit 1fd75ff.

The change is not correct and allows code to continue while it should
panic. For details, see:
#4360 (comment)

Author: aykevl

src/reflect/all_test.go

@@ -1102,20 +1102,6 @@ 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},
@@ -1133,7 +1119,6 @@ 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},
@@ -1155,7 +1140,6 @@ 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

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