fix: an incompatible behavior on map key decoder

fix #342
The map key decoder has an incompatible behavior when the type kind is string and the type has UnmarshalJSON.

decode_test.go

@@ -3889,3 +3889,21 @@ func TestIssue348(t *testing.T) {
 		}
 	}
 }
+
+type issue342 string
+
+func (t *issue342) UnmarshalJSON(b []byte) error {
+	panic("unreachable")
+}
+
+func TestIssue342(t *testing.T) {
+	var v map[issue342]int
+	in := []byte(`{"a":1}`)
+	if err := json.Unmarshal(in, &v); err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	expected := 1
+	if got := v["a"]; got != expected {
+		t.Errorf("unexpected result: got(%v) != expected(%v)", got, expected)
+	}
+}

internal/decoder/compile.go

@@ -154,6 +154,9 @@ func compileMapKey(typ *runtime.Type, structName, fieldName string, structTypeTo
 	if runtime.PtrTo(typ).Implements(unmarshalTextType) {
 		return newUnmarshalTextDecoder(runtime.PtrTo(typ), structName, fieldName), nil
 	}
+	if typ.Kind() == reflect.String {
+		return newStringDecoder(structName, fieldName), nil
+	}
 	dec, err := compile(typ, structName, fieldName, structTypeToDecoder)
 	if err != nil {
 		return nil, err