[Bug c++/109876] New: initializer_list not usable in constant expressions in a template

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109876

            Bug ID: 109876
           Summary: initializer_list not usable in constant expressions in
                    a template
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider this example:

#include <initializer_list>

template <int S>
struct Array {

};

#ifdef USE_TEMPLATE
template <int N>
#endif
struct Foo
{
  static constexpr std::initializer_list<int> num = { 1, 2 };

  Array<num.size()> ctx;

};

#ifdef USE_TEMPLATE
void f(Foo<5>) { }
#else
void f(Foo) { }
#endif

In all recent versions of gcc, compiling with -DUSE_TEMPLATE fails with an
error like:

<source>:15:17:   in 'constexpr' expansion of
'Foo<N>::num.std::initializer_list<int>::size()'
<source>:15:19: error: the value of 'Foo<N>::num' is not usable in a constant
expression
   15 |   Array<num.size()> ctx;
      |                   ^

This used to work in gcc 7 and gcc 8 (except 8.1), and I don't think there's
anything about Foo being a template that should prevent this from working. The
non-template case works on all gcc versions I've tried.