<feature>(IS): Use original errors.Is as final test for Is function

issue: Current is does not support custom Is definition on underlying errors
fix proposition: delegate the error check to standard errors.Is checker
so it does have expected behaviour on wrapped errors as well as custom
Is implementation

Author: tfreville

README.md

@@ -64,3 +64,6 @@ packages by Facebook and Dropbox, it was moved to one canonical location so
 everyone can benefit.
 
 This package is licensed under the MIT license, see LICENSE.MIT for details.
+
+## Changelog
+* v1.1.0 updated to use go1.13's standard-library errors.Is method instead of == in errors.Is

error.go

@@ -47,6 +47,7 @@ package errors
 
 import (
 	"bytes"
+	baseErrors "errors"
 	"fmt"
 	"reflect"
 	"runtime"
@@ -140,11 +141,10 @@ func WrapPrefix(e interface{}, prefix string, skip int) *Error {
 }
 
 // Is detects whether the error is equal to a given error. Errors
-// are considered equal by this function if they are the same object,
-// or if they both contain the same error inside an errors.Error.
+// are considered equal by this function if they are matched by errors.Is
+// or if their contained errors are matched through errors.Is
 func Is(e error, original error) bool {
-
-	if e == original {
+	if baseErrors.Is(e, original) {
 		return true
 	}
 

error_test.go

@@ -129,6 +129,50 @@ func TestIs(t *testing.T) {
 		t.Errorf("io.EOF is fmt.Errorf")
 	}
 
+	custErr := errorWithCustomIs{
+		Key: "TestForFun",
+		Err: io.EOF,
+	}
+
+	shouldMatch := errorWithCustomIs{
+		Key: "TestForFun",
+	}
+
+	shouldNotMatch := errorWithCustomIs{Key: "notOk"}
+
+	if !Is(custErr, shouldMatch) {
+		t.Errorf("custErr is not a TestForFun customError")
+	}
+
+	if Is(custErr, shouldNotMatch) {
+		t.Errorf("custErr is a notOk customError")
+	}
+
+	if !Is(custErr, New(shouldMatch)) {
+		t.Errorf("custErr is not a New(TestForFun customError)")
+	}
+
+	if Is(custErr, New(shouldNotMatch)) {
+		t.Errorf("custErr is a New(notOk customError)")
+	}
+
+	if !Is(New(custErr), shouldMatch) {
+		t.Errorf("New(custErr) is not a TestForFun customError")
+	}
+
+	if Is(New(custErr), shouldNotMatch) {
+		t.Errorf("New(custErr) is a notOk customError")
+	}
+
+	if !Is(New(custErr), New(shouldMatch)) {
+		t.Errorf("New(custErr) is not a New(TestForFun customError)")
+	}
+
+	if Is(New(custErr), New(shouldNotMatch)) {
+		t.Errorf("New(custErr) is a New(notOk customError)")
+	}
+
+
 }
 
 func TestWrapError(t *testing.T) {
@@ -317,3 +361,17 @@ func callersToFrames(callers []uintptr) []runtime.Frame {
 		}
 	}
 }
+
+type errorWithCustomIs struct {
+	Key string
+	Err error
+}
+
+func (ewci errorWithCustomIs) Error() string {
+	return "["+ewci.Key+"]: " + ewci.Err.Error()
+}
+
+func (ewci errorWithCustomIs) Is(target error) bool {
+	matched, ok := target.(errorWithCustomIs)
+	return ok && matched.Key == ewci.Key
+}