[Bug c++/59372] New: accepts-invalid with constexpr function pointer variable as non-type template argument

[richard-gccbugzilla at metafoo dot co.uk]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59372

            Bug ID: 59372
           Summary: accepts-invalid with constexpr function pointer
                    variable as non-type template argument
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk

Consider:

  void f() {}
  template<void (*)()> struct X {};
  template<void (&)()> struct Y {};

  constexpr void (*p)() = &f;
  X<p> a;

  constexpr void (*h())() { return &f; }
  X<h()> b;

  constexpr void (&q)() = f;
  X<q> c;

  constexpr void (&i())() { return f; }
  X<i()> d;

GCC correctly rejects the types of 'b', 'c', and 'd', but fails to reject 'a'.

All four are ill-formed by 14.3.2/1. In particular, a non-type template
argument for a parameter of pointer type must be either a null pointer, the
name of a non-type template-parameter, or an expression of the form '&
id-expression', and the '&' can only be omitted if the id-expression names a
function or array. (See also the discussion for core issue 1570, where this
direction was reaffirmed.)