encoding/json: fix UnmarshalTypeError without field & struct values

Fixes golang#26444

Author: bigflood

src/encoding/json/decode.go

@@ -672,6 +672,7 @@ func (d *decodeState) object(v reflect.Value) error {
 	}
 
 	var mapElem reflect.Value
+	originalErrorContext := d.errorContext
 
 	for {
 		// Read opening " of string key or closing }.
@@ -832,8 +833,7 @@ func (d *decodeState) object(v reflect.Value) error {
 			return errPhase
 		}
 
-		d.errorContext.Struct = ""
-		d.errorContext.Field = ""
+		d.errorContext = originalErrorContext
 	}
 	return nil
 }
@@ -991,7 +991,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 			if fromQuoted {
 				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
 			}
-			return &UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())}
+			return d.addErrorContext(&UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())})
 		case reflect.Interface:
 			n, err := d.convertNumber(s)
 			if err != nil {

src/encoding/json/decode_test.go

@@ -371,6 +371,10 @@ func (b *intWithPtrMarshalText) UnmarshalText(data []byte) error {
 	return (*intWithMarshalText)(b).UnmarshalText(data)
 }
 
+type mapStringToStringData struct {
+	Data map[string]string `json:"data"`
+}
+
 type unmarshalTest struct {
 	in                    string
 	ptr                   interface{}
@@ -866,6 +870,18 @@ var unmarshalTests = []unmarshalTest{
 		err:                   fmt.Errorf("json: unknown field \"extra\""),
 		disallowUnknownFields: true,
 	},
+	// issue 26444
+	// UnmarshalTypeError without field & struct values
+	{
+		in:  `{"data":{"test1": "bob", "test2": 123}}`,
+		ptr: new(mapStringToStringData),
+		err: &UnmarshalTypeError{Value: "number", Type: reflect.TypeOf(""), Offset: 37, Struct: "mapStringToStringData", Field: "data"},
+	},
+	{
+		in:  `{"data":{"test1": 123, "test2": "bob"}}`,
+		ptr: new(mapStringToStringData),
+		err: &UnmarshalTypeError{Value: "number", Type: reflect.TypeOf(""), Offset: 21, Struct: "mapStringToStringData", Field: "data"},
+	},
 }
 
 func TestMarshal(t *testing.T) {