[Bug c++/87786] New: Failed to mangle sizeof...(ArgPack) with template-alias captured arguments

[xecycle at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87786

            Bug ID: 87786
           Summary: Failed to mangle sizeof...(ArgPack) with
                    template-alias captured arguments
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xecycle at gmail dot com
  Target Milestone: ---

Reproducing code (at https://godbolt.org/z/dbqZu7):

```
#include <cstddef>

double g(int);

template <size_t X>
struct M {};

template <class... T>
using make_M = M<sizeof...(T)>;

template <class... T>
auto f(T&&... x)
  -> make_M<decltype(g(x))...>
{
  using _noop = int[];
  (void)_noop { ((void)x, 0)... };
  return {};
}

auto m = f(12, 34);
```

It fails to compile with versions >= 7.  At first it looks like a regression;
but it probably is not.  It's more like an incomplete attempt (in 7.x) to fix a
previous mangling bug.  6.x and earlier does compile, but the mangled output
used "sz" instead of "sP"; gcc/cp/mangle.c on trunk actually contains a line
`write_string ("sP");`, but seems it did not reach there in this example.