[Bug c++/80783] missing constexpr diagnostic for functions that cannot be constexpr

[victor.nawothnig at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80783

--- Comment #3 from Victor Nawothnig <victor.nawothnig at gmail dot com> ---
Created attachment 41384
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41384&action=edit
Examples

All testcases and their results attached for better readability