test: added ErrorAs

README.md

@@ -18,6 +18,8 @@ There are five key packages,
 - `portal` - utilities for allocating free ports for network listeners in tests
 
 ### Changes
+:ballot_box_with_check: v1.11.0 adds an ErrorAs helper
+
 :ballot_box_with_check: v1.10.0 adds a `util` package for helpers that return values
 
  - Adds ability to create and automatically clean up temporary files

examples_test.go

@@ -318,6 +318,17 @@ func ExampleErrorIs() {
 	// Output:
 }
 
+func ExampleErrorAs() {
+	e1 := errors.New("e1")
+	e2 := FakeError("foo")
+	e3 := errors.New("e3")
+	errorChain := errors.Join(e1, e2, e3)
+	var target FakeError
+	ErrorAs(t, errorChain, &target)
+	fmt.Println(target.Error())
+	// Output: foo
+}
+
 func ExampleFalse() {
 	False(t, 1 == int('a'))
 	// Output:

internal/assertions/assertions.go

@@ -162,6 +162,23 @@ func ErrorIs(err error, target error) (s string) {
 	return
 }
 
+func ErrorAs[E error, Target *E](err error, target Target) (s string) {
+	if err == nil {
+		s = "expected error; got nil\n"
+		return
+	}
+	if target == nil {
+		s = "expected target not to be nil"
+		return
+	}
+	if !errors.As(err, target) {
+		s = "expected errors.As match\n"
+		s += bullet(" error: %v\n", err)
+		s += bullet("target: %v\n", target)
+	}
+	return
+}
+
 func NoError(err error) (s string) {
 	if err != nil {
 		s = "expected nil error\n"

test.go

@@ -67,6 +67,13 @@ func ErrorIs(t T, err error, target error, settings ...Setting) {
 	invoke(t, assertions.ErrorIs(err, target), settings...)
 }
 
+// ErrorAs asserts err's tree contains an error that matches target.
+// If so, it sets target to the error value.
+func ErrorAs[E error, Target *E](t T, err error, target Target, settings ...Setting) {
+	t.Helper()
+	invoke(t, assertions.ErrorAs(err, target), settings...)
+}
+
 // NoError asserts err is a nil error.
 func NoError(t T, err error, settings ...Setting) {
 	t.Helper()

test_test.go

@@ -163,6 +163,38 @@ func TestErrorIs_nil(t *testing.T) {
 	ErrorIs(tc, nil, err)
 }
 
+type FakeError string
+
+func (e FakeError) Error() string {
+	return string(e)
+}
+
+func TestErrorAs(t *testing.T) {
+	tc := newCase(t, `expected errors.As match`)
+	t.Cleanup(tc.assert)
+
+	var target FakeError
+	e := errors.New("foo")
+	ErrorAs(tc, e, &target)
+}
+
+func TestErrorAs_nilErr(t *testing.T) {
+	tc := newCase(t, `expected error; got nil`)
+	t.Cleanup(tc.assert)
+
+	var target FakeError
+	ErrorAs(tc, nil, &target)
+}
+
+func TestErrorAs_nilTarget(t *testing.T) {
+	tc := newCase(t, `expected target not to be nil`)
+	t.Cleanup(tc.assert)
+
+	var target *FakeError
+	e := errors.New("foo")
+	ErrorAs(tc, e, target)
+}
+
 func TestNoError(t *testing.T) {
 	tc := newCase(t, `expected nil error`)
 	t.Cleanup(tc.assert)