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: 930cda5384
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>

src/encoding/json/decode_test.go

@@ -2572,3 +2572,34 @@ func TestUnmarshalMaxDepth(t *testing.T) {
 		}
 	}
 }
+
+func TestInvalidUnmarshalErrorIs(t *testing.T) {
+	err := fmt.Errorf("apackage: %w: failed to parse struct", &InvalidUnmarshalError{reflect.TypeOf("a")})
+	if !errors.Is(err, &InvalidUnmarshalError{}) {
+		t.Fatalf("%v should be unwrapped to a InvalidUnmarshalError", err)
+	}
+}
+
+func TestUnmarshalFieldErrorIs(t *testing.T) {
+	err := fmt.Errorf("apackage: %w: failed to parse struct", &UnmarshalFieldError{
+		Key:   "foo",
+		Type:  reflect.TypeOf("a"),
+		Field: reflect.StructField{Name: "b"},
+	})
+	if !errors.Is(err, &UnmarshalFieldError{}) {
+		t.Fatalf("%v should be unwrapped to a UnmarshalFieldError", err)
+	}
+}
+
+func TestUnmarshalTypeErrorIs(t *testing.T) {
+	err := fmt.Errorf("apackage: %w: failed to parse struct", &UnmarshalTypeError{
+		Value:  "foo",
+		Type:   reflect.TypeOf("a"),
+		Offset: 1,
+		Struct: "Foo",
+		Field:  "Bar",
+	})
+	if !errors.Is(err, &UnmarshalTypeError{}) {
+		t.Fatalf("%v should be unwrapped to a UnmarshalTypeError", err)
+	}
+}