[Bug c++/48452] New: [C++0x][SFINAE] Failures with n-ary initialization expressions (in return type)

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48452

           Summary: [C++0x][SFINAE] Failures with n-ary initialization
                    expressions (in return type)
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: daniel.krueg...@googlemail.com
                CC: ja...@redhat.com


Some n-ary initialization expressions in a SFINAE context used in a return type
definition either give hard errors:

  a) any T => abstract class
  b) cv void => T (T is a class type)

or give the wrong result:

  c) when void is initialized with more than one argument

#include <utility> // for declval

template<class T, class... Args>
decltype(T(std::declval<Args>()...), char()) f(int);

template<class, class...>
char (&f(...))[2];

struct A { virtual ~A() = 0; };
struct B {};

static_assert(sizeof(f<A, int, int>(0)) != 1, "Error"); // a
static_assert(sizeof(f<B, void, int>(0)) != 1, "Error"); // b
static_assert(sizeof(f<void, int, int>(0)) != 1, "Error"); // c

a) "error: cannot allocate an object of abstract type 'A' because the
following virtual functions are pure within 'A': virtual A::~A()"
b) "error: invalid use of void expression"
c) Assertion triggers!