[Bug c++/96170] New: Enhancement: allow requires-clause checking of statement expression substitution success or failure

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96170

            Bug ID: 96170
           Summary: Enhancement: allow requires-clause checking of
                    statement expression substitution success or failure
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wjwray at gmail dot com
  Target Milestone: ---

Some sort of sfinae on statement expressions containing structured bindings
can provide a convenient way to detect the number of bindings in a type.

Checking in requires clauses seems like the most direct approach:
https://godbolt.org/z/Eoq9rr

    template <typename T> constexpr int arity()
    {
           if constexpr (requires (T& a) { ({auto [x] = a;}); })   return 1;
      else if constexpr (requires (T& a) { ({auto [x,y] = a;}); }) return 2;
      //   ... etc ... 3,4,5 ... N
      else
           return 0;
    };

G++ accepts the template and, 
for a type T that decomposes into 1 element, arity<T>() == 1 succeeds.

However, it fails eagerly on the 1st condition for any other type,
e.g. arity<int>(), arity<int[2]>(), &etc...

c.f. bug 92402 seeking to match Clang's ability to sfinae in other contexts

(Clang accepts the template above but returns 0 for all T.)