encoding/json: merge FieldStack if the error's Field exists.

When people return UnmarshalTypeError in UnmarshalJSON,
we should append error's Field to FieldStack.

Fixes #68750

Author: j2gg0s

src/encoding/json/decode.go

@@ -255,7 +255,11 @@ func (d *decodeState) addErrorContext(err error) error {
 		switch err := err.(type) {
 		case *UnmarshalTypeError:
 			err.Struct = d.errorContext.Struct.Name()
-			err.Field = strings.Join(d.errorContext.FieldStack, ".")
+			fieldStack := d.errorContext.FieldStack
+			if err.Field != "" {
+				fieldStack = append(fieldStack, err.Field)
+			}
+			err.Field = strings.Join(fieldStack, ".")
 		}
 	}
 	return err

src/encoding/json/decode_test.go

@@ -62,6 +62,21 @@ func (*SS) UnmarshalJSON(data []byte) error {
 	return &UnmarshalTypeError{Value: "number", Type: reflect.TypeFor[SS]()}
 }
 
+type TAlias T
+
+func (tt *TAlias) UnmarshalJSON(data []byte) error {
+	t := T{}
+	if err := Unmarshal(data, &t); err != nil {
+		return err
+	}
+	*tt = TAlias(t)
+	return nil
+}
+
+type TOuter struct {
+	T TAlias
+}
+
 // ifaceNumAsFloat64/ifaceNumAsNumber are used to test unmarshaling with and
 // without UseNumber
 var ifaceNumAsFloat64 = map[string]any{
@@ -428,6 +443,7 @@ var unmarshalTests = []struct {
 	{CaseName: Name(""), in: `{"x": 1}`, ptr: new(tx), out: tx{}},
 	{CaseName: Name(""), in: `{"x": 1}`, ptr: new(tx), err: fmt.Errorf("json: unknown field \"x\""), disallowUnknownFields: true},
 	{CaseName: Name(""), in: `{"S": 23}`, ptr: new(W), out: W{}, err: &UnmarshalTypeError{"number", reflect.TypeFor[SS](), 0, "W", "S"}},
+	{CaseName: Name(""), in: `{"T": {"X": 23}}`, ptr: new(TOuter), out: TOuter{}, err: &UnmarshalTypeError{"number", reflect.TypeFor[string](), 0, "TOuter", "T.X"}},
 	{CaseName: Name(""), in: `{"F1":1,"F2":2,"F3":3}`, ptr: new(V), out: V{F1: float64(1), F2: int32(2), F3: Number("3")}},
 	{CaseName: Name(""), in: `{"F1":1,"F2":2,"F3":3}`, ptr: new(V), out: V{F1: Number("1"), F2: int32(2), F3: Number("3")}, useNumber: true},
 	{CaseName: Name(""), in: `{"k1":1,"k2":"s","k3":[1,2.0,3e-3],"k4":{"kk1":"s","kk2":2}}`, ptr: new(any), out: ifaceNumAsFloat64},