1 files changed, 77 insertions, 0 deletions
diff --git a/assert/errequal_test.go b/assert/errequal_test.go
new file mode 100644
index 0000000..5efbdc5
--- /dev/null
+++ b/assert/errequal_test.go
@@ -0,0 +1,77 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package assert_test
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"regexp"
+	"testing"
+
+	"go.xyrillian.de/gg/assert"
+	"go.xyrillian.de/gg/testcapture"
+)
+
+func TestErrEqual(t *testing.T) {
+	noError := error(nil)
+	fooError := errors.New("foo error")
+	barError := errors.New("bar error")
+	nestedFooError := fmt.Errorf("nested error: %w", fooError)
+
+	// test matching against nil
+	assert.ErrEqual(t, noError, nil)
+	assert.ErrEqual(t, noError, (*os.PathError)(nil))
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, fooError, nil)
+	}, `expected no error, but got "foo error"`)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, fooError, (*os.PathError)(nil))
+	}, `expected no error, but got "foo error"`)
+
+	// test matching against error
+	assert.ErrEqual(t, fooError, fooError)
+	assert.ErrEqual(t, nestedFooError, fooError)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, fooError, nestedFooError) // error nesting does not work the other way, we need to see the full `expected`error in`actual`
+	}, `expected "nested error: foo error", but got "foo error"`)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, barError, fooError) // check with unrelated errors for completeness
+	}, `expected "foo error", but got "bar error"`)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, nil, fooError)
+	}, `expected "foo error", but got no error`)
+
+	// test matching against string
+	assert.ErrEqual(t, fooError, "foo error")
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, nestedFooError, "foo error") // partial matches do not work
+	}, `expected "foo error", but got "nested error: foo error"`)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, barError, "foo error") // check with unrelated errors for completeness
+	}, `expected "foo error", but got "bar error"`)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, nil, "foo error")
+	}, `expected "foo error", but got no error`)
+
+	// test matching against regexp
+	fooRegexp := regexp.MustCompile(`foo`)
+	assert.ErrEqual(t, fooError, fooRegexp) // partial matches allowed here as long as regexp does not use ^ and $
+	assert.ErrEqual(t, nestedFooError, fooRegexp)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, barError, fooRegexp) // check with unrelated errors for completeness
+	}, `expected an error matching /foo/, but got "bar error"`)
+	expectErrors(t, func(t assert.TestingTB) {
+		assert.ErrEqual(t, nil, fooRegexp)
+	}, `expected an error matching /foo/, but got no error`)
+
+	// test matching against unexpected type
+	result := testcapture.Capture(t.Context(), t.Name(), func(t assert.TestingTB) {
+		assert.ErrEqual(t, errors.New("42"), 42)
+	})
+	assert.Equal(t, result, testcapture.Result{
+		Outcome: testcapture.OutcomePanicked,
+		Panic:   "cannot handle `expected` of type int",
+	})
+}