[Bug c++/71879] New: Error in unevaluated context breaks SFINAE

[manuel.schiller@bmw-carit.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71879

            Bug ID: 71879
           Summary: Error in unevaluated context breaks SFINAE
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: manuel.schiller@bmw-carit.de
  Target Milestone: ---

Having asked at Stackoverflow I was told the following might be a bug in GCC:

--------

When comparing member function pointers in an SFINAE context, member function
Foo::foo() exists, but its body contains code (x.hello()) which eventually does
not compile.

The following code compiles with clang. GCC however seems to evaluate the
function body of Foo::foo() and exits with an error ('struct Caller' has no
member named 'hello'), despite being in an unevaluated SFINAE context.


///
#include <iostream>
#include <type_traits>

struct Foo
{
    template <typename T> void foo(T&& x) { x.hello();}
};

struct Caller
{    
    template <typename T>
    auto call(T&& x, int) -> decltype(
        std::enable_if_t<
            std::is_same<
                decltype(&T::template foo<decltype(*this)>),
                void (T::*)(decltype(*this))
            >::value
        >())
    {
        //x.foo(*this);
    }

    template <typename T>
    void call(T&&, char){ std::cout << "hello" << std::endl;}
};

int main()
{
  Caller c;
  c.call(Foo(), 0);
}
///

Error message:

main.cpp: In instantiation of 'void Foo::foo(T&&) [with T = Caller&]':

main.cpp:14:14:   required by substitution of 'template<class T> decltype
(std::enable_if_t<std::is_same<decltype (& T:: foo<Caller&>), void
(T::*)(Caller&)>::value>()) Caller::call(T&&, int) [with T = Foo]'

main.cpp:31:18:   required from here

main.cpp:6:47: error: 'struct Caller' has no member named 'hello'

     template <typename T> void foo(T&& x) { x.hello(); }

                                             ~~^~~~~
------
version: g++ 6.1.0
flags: g++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp

------
Link to code on coliru: http://coliru.stacked-crooked.com/a/a984ad1ca2edcb0c

Link to stackoverflow: http://stackoverflow.com/q/38371924/678093