[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

[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]

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

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

[redi at gcc dot gnu.org]

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-03-28 19:58:41 UTC ---
(In reply to comment #0)
> that this happens because there is no constexpr conversion operator from the
> lambda function type to the corresponding function pointer type 

Is there supposed to be?

"The closure type for a lambda-expression with no lambda-capture has a public
non-virtual non-explicit const conversion function to pointer to function
having the same parameter and return types as the closure type's function call
operator. The value returned by this conversion function shall be the address
of a function that, when invoked, has the same effect as invoking the closure
type's function call operator."

A closure is not literal and its conversion operator isn't constexpr.

[Bug c++/52757] [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]

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

--- Comment #2 from Matt Calabrese <metaprogrammingtheworld at gmail dot com> 2012-03-28 20:10:02 UTC ---
Hmm. That seems artificially limiting, but thanks for looking it up -- sorry
for the erroneous bug report.

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

[redi at gcc dot gnu.org]

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

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-03-28 20:13:37 UTC ---
.