See also issue #8561.

http://play.golang.org/p/VYrNcjJ_Qo

compiles w/o a problem but leads to a guaranteed run-time failure: The method m in T1
and T2 have conflicting signatures and thus x (static type T1) cannot possibly be of
type T2.

The spec does not require that the signatures be conflict-free (
http://tip.golang.org/ref/spec#Type_assertions ):

"If T is an interface type, x.(T) asserts that the dynamic type of x implements the
interface T."

Interestingly, when it comes to interface assignments ("implements" relation),
conflicting method signatures are not permitted:

http://play.golang.org/p/QAZWsPqejr

prog.go:13: cannot use x2 (type T2) as type T1 in assignment:
    T2 does not implement T1 (wrong type for m method)
        have m(string)
        want m(int)
prog.go:14: cannot use x1 (type T1) as type T2 in assignment:
    T1 does not implement T2 (wrong type for m method)
        have m(int)
        want m(string)