Created attachment 27170 [details]
testcase

I just tested, the problem happens only if the template arguments of function_proxy are function pointers. More trivial types (int is what I've tested) just work fine

reduced:

template<typename T, T> class function_proxy;

template<typename Return, typename Obj, Return(*func)(Obj)>
struct function_proxy<Return(*)(Obj), func>
{
    static void wrapper(){ }
};

template<typename CT, CT> class member_helper;

template<typename Class, void(Class::*fun)()>
struct member_helper<void(Class::*)(), fun>
{
    static void as_free(Class& obj){ }

    static void worker(){
        //ERROR HERE: template argument 2 is invalid. Not very helpful message.
        (void) function_proxy<decltype(&as_free), &as_free>::wrapper;
    }
};

struct Test
{
    void test(){ }
};

int main()
{
    //does not work
    member_helper<decltype(&Test::test), &Test::test>::worker();
}

Fixed by r251438.

Author: mpolacek
Date: Fri Aug  2 17:51:53 2019
New Revision: 274027

URL: https://gcc.gnu.org/viewcvs?rev=274027&root=gcc&view=rev
Log:
	PR c++/53009
	* g++.dg/cpp0x/nontype3.C: New test.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/nontype3.C
Modified:
    trunk/gcc/testsuite/ChangeLog

Fixed.