From 97c9e230a3c46dfbb7a5feb9cfb3180f18f70b87 Mon Sep 17 00:00:00 2001
From: Sebastiaan van Stijn <github@gone.nl>
Date: Fri, 31 Jan 2025 21:58:12 +0100
Subject: [PATCH] add handling for Causer interface, and more coverage

- Add handling for custom errors that implement the Causer
  interface not not Wrapper.
- Add test-cases for multi-errors and customMessage.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
---
 errors.go      |  5 +++++
 errors_test.go | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/errors.go b/errors.go
index f654d19..c7f1b9f 100644
--- a/errors.go
+++ b/errors.go
@@ -417,6 +417,11 @@ func isInterface[T any](err error) bool {
 				}
 			}
 			return false
+		case interface{ Cause() error }:
+			err = x.Cause()
+			if err == nil {
+				return false
+			}
 		default:
 			return false
 		}
diff --git a/errors_test.go b/errors_test.go
index d4cb0d9..3bb0439 100644
--- a/errors_test.go
+++ b/errors_test.go
@@ -30,6 +30,10 @@ func TestInvalidArgument(t *testing.T) {
 		&errInvalidArgument{},
 		&customInvalidArgument{},
 		&wrappedInvalidArgument{errors.New("invalid parameter")},
+		&customMessage{err: ErrInvalidArgument, msg: "custom_message"},
+		&causerOnlyError{ErrInvalidArgument},
+		&unwrapperOnlyError{ErrInvalidArgument},
+		&multiError{errors.New("invalid parameter"), ErrNotFound, ErrInvalidArgument},
 	} {
 		if !IsInvalidArgument(match) {
 			t.Errorf("error did not match invalid argument: %#v", match)
@@ -203,3 +207,40 @@ func (*customInvalidArgument) InvalidParameter() {}
 type wrappedInvalidArgument struct{ error }
 
 func (*wrappedInvalidArgument) InvalidParameter() {}
+
+// causerOnlyError implements Causer interface, not Wrapper.
+type causerOnlyError struct {
+	error
+}
+
+func (c *causerOnlyError) Error() string {
+	return c.error.Error()
+}
+
+func (c *causerOnlyError) Cause() error {
+	return c.error
+}
+
+// unwrapperOnlyError implements Wrapper interface, not Causer.
+type unwrapperOnlyError struct {
+	error
+}
+
+func (c *unwrapperOnlyError) Error() string {
+	return c.error.Error()
+}
+
+func (c *unwrapperOnlyError) Unwrap() error {
+	return c.error
+}
+
+// multiError implements Wrapper interface, not Causer.
+type multiError []error
+
+func (c *multiError) Error() string {
+	return "multi error"
+}
+
+func (c *multiError) Unwrap() []error {
+	return *c
+}