[Bug c++/57466] New: Argument deduction fails for 'const T*' when T is function type

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57466

            Bug ID: 57466
           Summary: Argument deduction fails for 'const T*' when T is
                    function type
           Product: gcc
           Version: 4.7.3
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 57465

(This is the root cause of PR 57465, but I have a workaround for the library.)


template<typename T>
  constexpr bool
  is_pointer(const T*)
  { return true; }

template<typename T>
  constexpr bool
  is_pointer(const T&)
  { return false; }

using F = void();

constexpr F* f = nullptr;

static_assert( is_pointer(f), "function pointer is a pointer" );

G++ fails the static assertion:

t.cc:15:1: error: static assertion failed: function pointer is a pointer
 static_assert( is_pointer(f), "function pointer is a pointer" );
 ^

The first function template should be instantiated as bool is_pointer(F*) but
instead deduction fails because:
   types 'const T' and 'F {aka void()}' have incompatible cv-qualifiers


Note deduction succeeds with an explicit template argument:

template<typename T>
  constexpr bool
  is_pointer(const T*)
  { return true; }

using F = void();

constexpr F* f = nullptr;

constexpr bool pass = is_pointer<F>(f);
constexpr bool fail = is_pointer(f);