It's not more parentheses than in a method definition: func (a A) Foo(x int) (n int, err error) { … } Also your line isn't formatted properly. It should b

It's not more parentheses than in a method definition: func (a A) Foo(x int) (n int, err error) { … } Also your line isn't formatted properly. It should be: func Foo(type T)(t T) (t T, err error) { … }