[Bug c++/52757] New: [C++11] A lamba functions is not able to be used as a function pointer when passed as an explicit template argument

["metaprogrammingtheworld at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52757

             Bug #: 52757
           Summary: [C++11] A lamba functions is not able to be used as a
                    function pointer when passed as an explicit template
                    argument
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: metaprogrammingtheworld@gmail.com


The following are two simple test-cases for what appears to be the same
problem. The main issue is that you can't use a lambda function as a template
argument where the template parameter type is a function pointer type. It seems
that this happens because there is no constexpr conversion operator from the
lambda function type to the corresponding function pointer type (in places
where the pointer is not required to be a compile-time constant, the conversion
works properly, as expected).

First test-case:

//////////
template< int (*)() > struct foo;
typedef foo< [] { return 0; } > bar;
//////////

main.cpp:2:31: error: could not convert template argument '{}' to 'int (*)()'
main.cpp:2:36: error: invalid type in declaration before ';' token



Second test-case (error message may shed light on the root of the problem):

//////////
constexpr int (*foo)() = [] { return 0; };
//////////

main.cpp:1:41: error: '<lambda()>::operator int (*)()() const' is not a
constexpr function