[Bug c++/91073] [9/10 Regression] if constexpr no longer works directly with Concepts

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91073

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2019-11-19 00:00:00         |2019-12-3

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #6)
> The bug only seems to happen when the concept is variadic.

Or has a default argument, as in the dup I just reported:

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 it with -std=gnu++14 -fconcepts (since r276764) but still rejects
it with -std=gnu++17 -fconcepts or -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
      |                    ^