feat: allow use of providers that don't return errors

callbacks.go

@@ -34,8 +34,17 @@ func (b bindings) addTo(impl, iface any) {
 func (b bindings) addProvider(provider any) error {
 	pv := reflect.ValueOf(provider)
 	t := pv.Type()
-	if t.Kind() != reflect.Func || t.NumOut() != 2 || t.Out(1) != reflect.TypeOf((*error)(nil)).Elem() {
-		return fmt.Errorf("%T must be a function with the signature func(...)(T, error)", provider)
+	if t.Kind() != reflect.Func {
+		return fmt.Errorf("%T must be a function", provider)
+	}
+
+	if t.NumOut() == 0 {
+		return fmt.Errorf("%T must be a function with the signature func(...)(T, error) or func(...) T", provider)
+	}
+	if t.NumOut() == 2 {
+		if t.Out(1) != reflect.TypeOf((*error)(nil)).Elem() {
+			return fmt.Errorf("missing error; %T must be a function with the signature func(...)(T, error) or func(...) T", provider)
+		}
 	}
 	rt := pv.Type().Out(0)
 	b[rt] = provider
@@ -143,7 +152,7 @@ func callAnyFunction(f reflect.Value, bindings bindings) (out []any, err error)
 		if err != nil {
 			return nil, fmt.Errorf("%s: %w", pt, err)
 		}
-		if ferrv := reflect.ValueOf(argv[len(argv)-1]); ferrv.IsValid() && !ferrv.IsNil() {
+		if ferrv := reflect.ValueOf(argv[len(argv)-1]); ferrv.IsValid() && ferrv.Type().Implements(callbackReturnSignature) && !ferrv.IsNil() {
 			return nil, ferrv.Interface().(error) //nolint:forcetypeassert
 		}
 		in = append(in, reflect.ValueOf(argv[0]))