Add ErrorIs and ErrorAs (#105)

Author: FollowTheProcess

.golangci.yml

@@ -58,6 +58,13 @@ linters:
           - gosec # Tests don't need security stuff
           - goconst # Nah
 
+      # test.ErrorAs returns the matched error for optional chained assertions;
+      # discarding it is a supported pattern. errcheck's exclude-functions
+      # doesn't currently match generic instantiations, so match by source instead.
+      - source: 'test\.ErrorAs\['
+        linters:
+          - errcheck
+
   settings:
     cyclop:
       max-complexity: 20

README.md

@@ -197,6 +197,32 @@ func TestOutput(t *testing.T) {
 
 Under the hood `CaptureOutput` temporarily captures both streams, copies the data to a buffer and returns the output back to you, before cleaning everything back up again.
 
+### A note on `ErrorAs` and `errcheck`
+
+`test.ErrorAs[T]` returns the matched error so you can chain further assertions on its fields:
+
+```go
+got := test.ErrorAs[*os.PathError](t, err)
+test.Equal(t, got.Op, "open")
+```
+
+The return value is optional — discarding it is a supported pattern when you only want the type-check assertion:
+
+```go
+test.ErrorAs[*os.PathError](t, err) // pure type check, return ignored
+```
+
+If you lint with [`errcheck`] it will flag the discard because `T` is constrained to `error`. `errcheck`'s `exclude-functions` doesn't currently match generic instantiations, so the cleanest fix is a source-based exclusion in `.golangci.yml`:
+
+```yaml
+linters:
+  exclusions:
+    rules:
+      - source: 'test\.ErrorAs\['
+        linters:
+          - errcheck
+```
+
 ### See Also
 
 - [FollowTheProcess/snapshot] for golden file/snapshot testing 📸
@@ -205,6 +231,7 @@ Under the hood `CaptureOutput` temporarily captures both streams, copies the dat
 
 This package was created with [copier] and the [FollowTheProcess/go_copier] project template.
 
+[`errcheck`]: https://github.com/kisielk/errcheck
 [copier]: https://copier.readthedocs.io/en/stable/
 [FollowTheProcess/go_copier]: https://github.com/FollowTheProcess/go_copier
 [matryer/is]: https://github.com/matryer/is

test.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"math"
 	"os"
+	"reflect"
 	"strings"
 	"testing"
 
@@ -300,6 +301,85 @@ func Err(tb testing.TB, err error, options ...Option) {
 	}
 }
 
+// ErrorIs fails if err does not match target as reported by [errors.Is].
+//
+//	var ErrMadeUp = errors.New("made up error")
+//	test.ErrorIs(t, err, ErrMadeUp)
+func ErrorIs(tb testing.TB, err, target error, options ...Option) {
+	tb.Helper()
+
+	cfg := defaultConfig()
+	cfg.title = "Wrong Error"
+
+	for _, option := range options {
+		if optionErr := option.apply(&cfg); optionErr != nil {
+			tb.Fatalf("ErrorIs: could not apply options: %v", optionErr)
+
+			return
+		}
+	}
+
+	if !errors.Is(err, target) {
+		fail := failure[error]{
+			got:  err,
+			want: target,
+			cfg:  cfg,
+		}
+		tb.Fatal(fail.String())
+	}
+}
+
+// ErrorAs asserts that err or some error in its chain matches the concrete
+// type T as reported by [errors.AsType], and returns the matched error so
+// the caller can make further assertions on its fields without having to
+// unwrap it a second time. The return value may be ignored.
+//
+// Discard the return when you only care about the type check:
+//
+//	test.ErrorAs[*os.PathError](t, err)
+//
+// Or bind it to drill into the matched error:
+//
+//	got := test.ErrorAs[*os.PathError](t, err)
+//	test.Equal(t, got.Op, "open")
+//	test.Equal(t, got.Path, "/does/not/exist")
+func ErrorAs[T error](tb testing.TB, err error, options ...Option) T {
+	tb.Helper()
+
+	cfg := defaultConfig()
+	cfg.title = "Wrong Error Type"
+
+	for _, option := range options {
+		if optionErr := option.apply(&cfg); optionErr != nil {
+			tb.Fatalf("ErrorAs: could not apply options: %v", optionErr)
+
+			var zero T
+
+			return zero
+		}
+	}
+
+	if target, ok := errors.AsType[T](err); ok {
+		return target
+	}
+
+	got := "<nil>"
+	if err != nil {
+		got = fmt.Sprintf("%T: %s", err, err.Error())
+	}
+
+	fail := failure[string]{
+		got:  got,
+		want: fmt.Sprintf("error matching %s", reflect.TypeFor[T]()),
+		cfg:  cfg,
+	}
+	tb.Fatal(fail.String())
+
+	var zero T
+
+	return zero
+}
+
 // WantErr fails if you got an error and didn't want it, or if you didn't
 // get an error but wanted one.
 //

test_test.go

@@ -325,6 +325,104 @@ func TestTest(t *testing.T) {
 			},
 			wantFail: true,
 		},
+		{
+			name: "ErrorIs/pass",
+			fn: func(tb testing.TB) {
+				sentinel := errors.New("sentinel")
+				test.ErrorIs(tb, sentinel, sentinel)
+			},
+			wantFail: false,
+		},
+		{
+			name: "ErrorIs/pass wrapped",
+			fn: func(tb testing.TB) {
+				sentinel := errors.New("sentinel")
+				wrapped := fmt.Errorf("while frobnicating: %w", sentinel)
+				test.ErrorIs(tb, wrapped, sentinel)
+			},
+			wantFail: false,
+		},
+		{
+			name: "ErrorIs/fail",
+			fn: func(tb testing.TB) {
+				test.ErrorIs(tb, errors.New("bang"), errors.New("not bang"))
+			},
+			wantFail: true,
+		},
+		{
+			name: "ErrorIs/fail nil",
+			fn: func(tb testing.TB) {
+				test.ErrorIs(tb, nil, errors.New("wanted this one"))
+			},
+			wantFail: true,
+		},
+		{
+			name: "ErrorIs/fail with context",
+			fn: func(tb testing.TB) {
+				test.ErrorIs(tb, errors.New("bang"), errors.New("not bang"), test.Context("Expected the other error"))
+			},
+			wantFail: true,
+		},
+		{
+			name: "ErrorIs/fail with title",
+			fn: func(tb testing.TB) {
+				test.ErrorIs(tb, errors.New("bang"), errors.New("not bang"), test.Title("Wrong one"))
+			},
+			wantFail: true,
+		},
+		{
+			name: "ErrorAs/pass",
+			fn: func(tb testing.TB) {
+				boom := &inputError{msg: "boom"}
+
+				got := test.ErrorAs[*inputError](tb, boom)
+				if got != boom {
+					tb.Fatal("ErrorAs did not return the matched error")
+				}
+			},
+			wantFail: false,
+		},
+		{
+			name: "ErrorAs/pass wrapped",
+			fn: func(tb testing.TB) {
+				inner := &inputError{msg: "boom"}
+				wrapped := fmt.Errorf("while frobnicating: %w", inner)
+
+				got := test.ErrorAs[*inputError](tb, wrapped)
+				if got != inner {
+					tb.Fatal("ErrorAs did not return the wrapped error")
+				}
+			},
+			wantFail: false,
+		},
+		{
+			name: "ErrorAs/fail",
+			fn: func(tb testing.TB) {
+				test.ErrorAs[*inputError](tb, &outputError{msg: "nope"})
+			},
+			wantFail: true,
+		},
+		{
+			name: "ErrorAs/fail nil",
+			fn: func(tb testing.TB) {
+				test.ErrorAs[*inputError](tb, nil)
+			},
+			wantFail: true,
+		},
+		{
+			name: "ErrorAs/fail with context",
+			fn: func(tb testing.TB) {
+				test.ErrorAs[*inputError](tb, &outputError{msg: "nope"}, test.Context("Expected an inputError"))
+			},
+			wantFail: true,
+		},
+		{
+			name: "ErrorAs/fail with title",
+			fn: func(tb testing.TB) {
+				test.ErrorAs[*inputError](tb, &outputError{msg: "nope"}, test.Title("Type mismatch"))
+			},
+			wantFail: true,
+		},
 		{
 			name: "WantErr/pass error",
 			fn: func(tb testing.TB) {
@@ -621,3 +719,14 @@ func TestCapture(t *testing.T) {
 		test.Equal(t, stderr, "")
 	})
 }
+
+// inputError is a concrete error type used to exercise test.ErrorAs.
+type inputError struct{ msg string }
+
+func (e *inputError) Error() string { return e.msg }
+
+// outputError is an unrelated concrete error type used to exercise the
+// "wrong type" failure path of test.ErrorAs.
+type outputError struct{ msg string }
+
+func (e *outputError) Error() string { return e.msg }

testdata/snapshots/TestTest/ErrorAs/fail.snap

@@ -0,0 +1,10 @@
+source: test_test.go
+expression: buf.String()
+---
+|
+
+  Wrong Error Type
+  ----------------
+
+  Got:	*test_test.outputError: nope
+  Wanted:	error matching *test_test.inputError

testdata/snapshots/TestTest/ErrorAs/fail_nil.snap

@@ -0,0 +1,10 @@
+source: test_test.go
+expression: buf.String()
+---
+|
+
+  Wrong Error Type
+  ----------------
+
+  Got:	<nil>
+  Wanted:	error matching *test_test.inputError

testdata/snapshots/TestTest/ErrorAs/fail_with_context.snap

@@ -0,0 +1,12 @@
+source: test_test.go
+expression: buf.String()
+---
+|
+
+  Wrong Error Type
+  ----------------
+
+  Got:	*test_test.outputError: nope
+  Wanted:	error matching *test_test.inputError
+
+  (Expected an inputError)

testdata/snapshots/TestTest/ErrorAs/fail_with_title.snap

@@ -0,0 +1,10 @@
+source: test_test.go
+expression: buf.String()
+---
+|
+
+  Type mismatch
+  -------------
+
+  Got:	*test_test.outputError: nope
+  Wanted:	error matching *test_test.inputError

testdata/snapshots/TestTest/ErrorIs/fail.snap

@@ -0,0 +1,10 @@
+source: test_test.go
+expression: buf.String()
+---
+|
+
+  Wrong Error
+  -----------
+
+  Got:	bang
+  Wanted:	not bang

testdata/snapshots/TestTest/ErrorIs/fail_nil.snap

@@ -0,0 +1,10 @@
+source: test_test.go
+expression: buf.String()
+---
+|
+
+  Wrong Error
+  -----------
+
+  Got:	<nil>
+  Wanted:	wanted this one