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From: "ai.azuma at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/56913] New: [C++11] SFINAE for ill-formed pointer-to-member operators with incompatible ref-qualifiers
Date: Thu, 11 Apr 2013 00:55:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56913

             Bug #: 56913
           Summary: [C++11] SFINAE for ill-formed pointer-to-member
                    operators with incompatible ref-qualifiers
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ai.azuma@gmail.com


Created attachment 29851
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29851
Output of -v option and preprocessed source

GCC 4.8.1 20130404 and 4.9.0 20130407 reject the following well-formed code;

//-----------------------------------------------------------
template<typename T>
T &&declval();

template<typename T, typename U,
         typename = decltype((declval<T>().*declval<U>())())>
constexpr bool test(int)
{
  return true;
}

template<typename T, typename U>
constexpr bool test(...)
{
  return false;
}

struct S
{};

static_assert(!test<S, void (S::*)() &>(0), "");
static_assert(test<S, void (S::*)() &&>(0), "");
static_assert(test<S &, void (S::*)() &>(0), "");
static_assert(!test<S &, void (S::*)() &&>(0), "");
//-----------------------------------------------------------

For the above code, GCC 4.9.0 20130407 complains as follows;

main.cpp:5:43: error: pointer-to-member-function type 'void (S::*)() &'
requires an lvalue
          typename = decltype((declval<T>().*declval<U>())())>
                                           ^
main.cpp:20:1: error: static assertion failed: 
 static_assert(!test<S, void (S::*)() &>(0), "");
 ^
main.cpp:5:43: error: pointer-to-member-function type 'void (S::*)() &&'
requires an rvalue
          typename = decltype((declval<T>().*declval<U>())())>
                                           ^
main.cpp:23:1: error: static assertion failed: 
 static_assert(!test<S &, void (S::*)() &&>(0), "");
 ^

SFINAE does not seem to be applied to ill-formed pointer-to-member operators
(as per N3485 [expr.mptr.oper]/6) instantiated in the first and fourth
static_asserts, and they result in hard errors.