encoding/json: add Path to UnmarshalTypeError

When parsing nested object, UnmarshalTypeError does not contain actual
path to nested field in original JSON.

This commit change Field to contain the full path to that field. One
can get the Field name by stripping all the leading path elements.

Fixes #22369

Change-Id: I6969cc08abe8387a351e3fb2944adfaa0dccad2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/145218Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/decode.go

@@ -125,7 +125,7 @@ type UnmarshalTypeError struct {
 	Type   reflect.Type // type of Go value it could not be assigned to
 	Offset int64        // error occurred after reading Offset bytes
 	Struct string       // name of the struct type containing the field
-	Field  string       // name of the field holding the Go value
+	Field  string       // the full path from root node to the field
 }
 
 func (e *UnmarshalTypeError) Error() string {
@@ -730,7 +730,11 @@ func (d *decodeState) object(v reflect.Value) error {
 					}
 					subv = subv.Field(i)
 				}
+				if originalErrorContext.Field == "" {
 					d.errorContext.Field = f.name
+				} else {
+					d.errorContext.Field = originalErrorContext.Field + "." + f.name
+				}
 				d.errorContext.Struct = t
 			} else if d.disallowUnknownFields {
 				d.saveError(fmt.Errorf("json: unknown field %q", key))

src/encoding/json/decode_test.go

@@ -45,6 +45,14 @@ type W struct {
 	S SS
 }
 
+type P struct {
+	PP PP
+}
+
+type PP struct {
+	T T
+}
+
 type SS string
 
 func (*SS) UnmarshalJSON(data []byte) error {
@@ -816,7 +824,7 @@ var unmarshalTests = []unmarshalTest{
 		err: &UnmarshalTypeError{
 			Value:  "string",
 			Struct: "V",
-			Field:  "F2",
+			Field:  "V.F2",
 			Type:   reflect.TypeOf(int32(0)),
 			Offset: 20,
 		},
@@ -827,7 +835,7 @@ var unmarshalTests = []unmarshalTest{
 		err: &UnmarshalTypeError{
 			Value:  "string",
 			Struct: "V",
-			Field:  "F2",
+			Field:  "V.F2",
 			Type:   reflect.TypeOf(int32(0)),
 			Offset: 30,
 		},
@@ -923,6 +931,18 @@ var unmarshalTests = []unmarshalTest{
 		ptr: new(MustNotUnmarshalText),
 		err: &UnmarshalTypeError{Value: "object", Type: reflect.TypeOf(&MustNotUnmarshalText{}), Offset: 1},
 	},
+	// #22369
+	{
+		in:  `{"PP": {"T": {"Y": "bad-type"}}}`,
+		ptr: new(P),
+		err: &UnmarshalTypeError{
+			Value:  "string",
+			Struct: "T",
+			Field:  "PP.T.Y",
+			Type:   reflect.TypeOf(int(0)),
+			Offset: 29,
+		},
+	},
 }
 
 func TestMarshal(t *testing.T) {