Fix encoding of using empty interface as map key

encode_test.go

@@ -1912,3 +1912,9 @@ func TestIssue235(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestEncodeMapKeyTypeInterface(t *testing.T) {
+	if _, err := json.Marshal(map[interface{}]interface{}{"a": 1}); err == nil {
+		t.Fatal("expected error")
+	}
+}

internal/encoder/compiler.go

@@ -486,8 +486,6 @@ func compileKey(ctx *compileContext) (*Opcode, error) {
 	switch typ.Kind() {
 	case reflect.Ptr:
 		return compilePtr(ctx)
-	case reflect.Interface:
-		return compileInterface(ctx)
 	case reflect.String:
 		return compileString(ctx)
 	case reflect.Int: