gcc support for C++20 P0929 (Checking for abstract class types)

[Tom Honermann]

Gcc (trunk) appears to have added support for P0929 
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0929r2.html>, 
but the C++20 support page 
<https://gcc.gnu.org/projects/cxx-status.html#cxx20> doesn't acknowledge 
it.  Oversight?

https://www.godbolt.org/z/a4EjaG

Tom.