encoding/json: implement Is on all errors

Allows users to check:

      errors.Is(err, &UnmarshalTypeError{})
      errors.Is(err, &UnmarshalFieldError{})
      errors.Is(err, &InvalidUnmarshalError{})
      errors.Is(err, &UnsupportedValueError{})
      errors.Is(err, &MarshalerError{})

which is the recommended way of checking for kinds of errors.

SyntaxError.Is was implemented in CL 253037.
As and Unwrap relevant methods will be added in future CLs.

Change-Id: I1f8a503b8fdc0f3afdfe9669a91f3af8d960e028
GitHub-Last-Rev: 930cda5384c987a0b31f277ba3b4ab690ea74ac3
GitHub-Pull-Request: golang/go#41360
Reviewed-on: https://go-review.googlesource.com/c/go/+/254537
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>

1 files changed, 23 insertions, 1 deletions

diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index 7290eca06f..90826a7f47 100644
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -7,6 +7,7 @@ package json
 import (
 	"bytes"
 	"encoding"
+	"errors"
 	"fmt"
 	"log"
 	"math"
@@ -211,7 +212,7 @@ var unsupportedValues = []interface{}{
 func TestUnsupportedValues(t *testing.T) {
 	for _, v := range unsupportedValues {
 		if _, err := Marshal(v); err != nil {
-			if _, ok := err.(*UnsupportedValueError); !ok {
+			if !errors.Is(err, &UnsupportedValueError{}) {
 				t.Errorf("for %v, got %T want UnsupportedValueError", v, err)
 			}
 		} else {
@@ -1155,3 +1156,24 @@ func TestMarshalerError(t *testing.T) {
 		}
 	}
 }
+
+func TestMarshalerErrorIs(t *testing.T) {
+	err := fmt.Errorf("apackage: %w: failed to parse struct", &MarshalerError{
+		reflect.TypeOf("a"),
+		fmt.Errorf("something"),
+		"TestMarshalerErrorIs",
+	})
+	if !errors.Is(err, &MarshalerError{}) {
+		t.Fatalf("%v should be unwrapped to a MarshalerError", err)
+	}
+}
+
+func TestUnsupportedValueErrorIs(t *testing.T) {
+	err := fmt.Errorf("apackage: %w: failed to parse struct", &UnsupportedValueError{
+		Value: reflect.Value{},
+		Str:   "Foo",
+	})
+	if !errors.Is(err, &UnsupportedValueError{}) {
+		t.Fatalf("%v should be unwrapped to a UnsupportedValueError", err)
+	}
+}