In Go1.9, with type alias support (currently on dev.typealias branch):

type T struct{}
type A = T
func (T) m() {}
func (A) m() {} // <<<< this should be an error
func (A) m() {} // error reported here

compiles without reporting an error for the first method A.m.

The underlying problem is that the gc compiler simply throws in all method declarations into the package "scope" with artificial names T.m, (*T).m, A.m, so (at least during the declaration phase) T.m and A.m have different names and won't lead to a redeclaration error (see dcl.go, func methodname0). The second A.m declaration leads to an error because it conflicts with the first one.

This also applies to redeclaration of a method m once on a T receiver and once on a *T receiver; however that redeclaration is caught later (see dcl.go, func addmethod).

We need to do something similar for methods declared on receivers that are type aliases.

Independent of aliases, it's odd that some method redeclarations are detected during node construction while others are detected later (with different error messages).