[Bug c++/106838] Built-in traits have wrong preconditions

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106838

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh, and actually __is_final is a fourth kind:

// D.
// "If T is a class type, T shall be a complete type."

So the same as C, but doesn't allow incomplete unions.