[Bug c++/92771] [9/10 Regression] Concept won't use default template argument

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92771

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

template<typename P, typename Arghhh = void>
concept one_or_two = true;

template<typename P>
concept one = one_or_two<P>;

template<typename T>
constexpr void
foo()
{
  if (one<T>) // OK
  { }

  if (one_or_two<T>) // ERROR
  { }
}

gcc-8-branch accepts this with -fconcepts.
gcc-9-branch rejects it with -fconcepts

92771.cc: In function 'constexpr void foo()':
92771.cc:14:20: error: expected unqualified-id before ')' token
   14 |   if (one_or_two<T>) // ERROR
      |                    ^


trunk accepts with -fconcepts (?!?) but not with -std=gnu++2a:

92771.cc: In function 'constexpr void foo()':
92771.cc:14:7: error: expected 'auto' or 'decltype(auto)' after 'one_or_two<T>'
   14 |   if (one_or_two<T>) // ERROR
      |       ^~~~~~~~~~~~~
92771.cc:14:20: error: expected unqualified-id before ')' token
   14 |   if (one_or_two<T>) // ERROR
      |                    ^