implement ".Unwrap() error" on Error type (gin-gonic#2525)

errors.go

@@ -90,6 +90,11 @@ func (msg *Error) IsType(flags ErrorType) bool {
 	return (msg.Type & flags) > 0
 }
 
+// Unwrap returns the wrapped error, to allow interoperability with errors.Is(), errors.As() and errors.Unwrap()
+func (msg *Error) Unwrap() error {
+	return msg.Err
+}
+
 // ByType returns a readonly copy filtered the byte.
 // ie ByType(gin.ErrorTypePublic) returns a slice of errors with type=ErrorTypePublic.
 func (a errorMsgs) ByType(typ ErrorType) errorMsgs {

errors_1.13_test.go

@@ -0,0 +1,33 @@
+// +build go1.13
+
+package gin
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type TestErr string
+
+func (e TestErr) Error() string { return string(e) }
+
+// TestErrorUnwrap tests the behavior of gin.Error with "errors.Is()" and "errors.As()".
+// "errors.Is()" and "errors.As()" have been added to the standard library in go 1.13,
+// hence the "// +build go1.13" directive at the beginning of this file.
+func TestErrorUnwrap(t *testing.T) {
+	innerErr := TestErr("somme error")
+
+	// 2 layers of wrapping : use 'fmt.Errorf("%w")' to wrap a gin.Error{}, which itself wraps innerErr
+	err := fmt.Errorf("wrapped: %w", &Error{
+		Err:  innerErr,
+		Type: ErrorTypeAny,
+	})
+
+	// check that 'errors.Is()' and 'errors.As()' behave as expected :
+	assert.True(t, errors.Is(err, innerErr))
+	var testErr TestErr
+	assert.True(t, errors.As(err, &testErr))
+}