encoding/json: reduce error text regressions under goexperiment.jsonv2

There were minor and unnecessary error text changes
when v1 was implemented using v2.
Reduce divergences if possible.

Of the cases reported in #74713, there are no more differences for:

	v1: json: cannot unmarshal number into Go value of type chan int
	v2: json: cannot unmarshal number into Go value of type chan int

and

	v1: json: cannot unmarshal number into Go value of type error
	v2: json: cannot unmarshal number into Go value of type error

However, there is a difference between:

	v1: json: cannot unmarshal string into Go struct field .F.V of type int
	v2: json: cannot unmarshal string into Go struct field S.F.V of type int

For reasons unclear, the v1 logic was always inconsistent about
whether it could properly record the root struct type,
while the v1 emulation layer under v2 is always able to.

This only modifies code that is compiled in under goexperiment.jsonv2.

Fixes #74713

Change-Id: I9e87323b1810130cb929288fdd86aff4be82d5f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/689918
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/src/encoding/json/decode_test.go b/src/encoding/json/decode_test.go
index 0df31c82c8..d12495f90b 100644
--- a/src/encoding/json/decode_test.go
+++ b/src/encoding/json/decode_test.go
@@ -416,6 +416,8 @@ type DoublePtr struct {
 	J **int
 }
 
+type NestedUnamed struct{ F struct{ V int } }
+
 var unmarshalTests = []struct {
 	CaseName
 	in                    string
@@ -1213,6 +1215,28 @@ var unmarshalTests = []struct {
 			F string `json:"-,omitempty"`
 		}{"hello"},
 	},
+
+	{
+		CaseName: Name("ErrorForNestedUnamed"),
+		in:       `{"F":{"V":"s"}}`,
+		ptr:      new(NestedUnamed),
+		out:      NestedUnamed{},
+		err:      &UnmarshalTypeError{Value: "string", Type: reflect.TypeFor[int](), Offset: 13, Field: "F.V"},
+	},
+	{
+		CaseName: Name("ErrorInterface"),
+		in:       `1`,
+		ptr:      new(error),
+		out:      error(nil),
+		err:      &UnmarshalTypeError{Value: "number", Type: reflect.TypeFor[error](), Offset: 1},
+	},
+	{
+		CaseName: Name("ErrorChan"),
+		in:       `1`,
+		ptr:      new(chan int),
+		out:      (chan int)(nil),
+		err:      &UnmarshalTypeError{Value: "number", Type: reflect.TypeFor[chan int](), Offset: 1},
+	},
 }
 
 func TestMarshal(t *testing.T) {