check: Add assert.ErrIs

check/assert.go

@@ -1,6 +1,7 @@
 package check
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
 	"runtime/debug"
@@ -466,6 +467,41 @@ func (a assert) NotNilf(g any, format string, args ...any) bool {
 	return true
 }
 
+func (a assert) failErrIs(err, target error, format string, args ...any) {
+	a.helper()
+	a.error(
+		fmt.Sprintf(format, args...),
+		fmt.Sprintf(""+
+			"Expected error: %q\n"+
+			"           got: %s",
+			err,
+			target,
+		),
+	)
+}
+
+// ErrIs ensures that [errors.Is] returns true.
+func (a assert) ErrIs(err, target error) bool {
+	if !errors.Is(err, target) {
+		a.helper()
+		a.failErrIs(err, target, "")
+		return false
+	}
+
+	return true
+}
+
+// ErrIsf ensures that [errors.Is] returns true.
+func (a assert) ErrIsf(err, target error, format string, args ...any) bool {
+	if !errors.Is(err, target) {
+		a.helper()
+		a.failErrIs(err, target, format, args...)
+		return false
+	}
+
+	return true
+}
+
 type paniced struct {
 	recovered any
 	stack     string

check/assert_test.go

@@ -3,6 +3,7 @@ package check
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"testing"
 )
 
@@ -517,6 +518,28 @@ func TestAssertNotNil(t *testing.T) {
 	)
 }
 
+func TestAssertErrIs(t *testing.T) {
+	ta := newTestAssert(t)
+
+	ta.check(
+		ta.ErrIs(fs.ErrClosed, fs.ErrClosed),
+		true,
+	)
+	ta.check(
+		ta.ErrIsf(fs.ErrClosed, fs.ErrClosed, "fmt %d", 1),
+		true,
+	)
+
+	ta.check(
+		ta.ErrIs(errors.New("test"), fs.ErrClosed),
+		false,
+	)
+	ta.check(
+		ta.ErrIsf(errors.New("test"), fs.ErrClosed, "fmt %d", 1),
+		false,
+	)
+}
+
 func TestAssertPanics(t *testing.T) {
 	ta := newTestAssert(t)