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, 12 insertions, 0 deletions

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 578d551102..8e6b342b59 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -245,6 +245,12 @@ func (e *UnsupportedValueError) Error() string {
 	return "json: unsupported value: " + e.Str
 }
 
+// Is returns true if target is a UnsupportedValueError.
+func (e *UnsupportedValueError) Is(target error) bool {
+	_, ok := target.(*UnsupportedValueError)
+	return ok
+}
+
 // Before Go 1.2, an InvalidUTF8Error was returned by Marshal when
 // attempting to encode a string value with invalid UTF-8 sequences.
 // As of Go 1.2, Marshal instead coerces the string to valid UTF-8 by
@@ -279,6 +285,12 @@ func (e *MarshalerError) Error() string {
 // Unwrap returns the underlying error.
 func (e *MarshalerError) Unwrap() error { return e.Err }
 
+// Is returns true if target is a MarshalerError.
+func (e *MarshalerError) Is(target error) bool {
+	_, ok := target.(*MarshalerError)
+	return ok
+}
+
 var hex = "0123456789abcdef"
 
 // An encodeState encodes JSON into a bytes.Buffer.