[Bug c++/115792] throw() should be diagnostic/errored out with C++20+

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115792

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|GCC accepts [] throw () {}  |throw() should be
                   |                            |diagnostic/errored out with
                   |                            |C++20+
             Status|REOPENED                    |NEW
           Keywords|                            |accepts-invalid

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So let's change this to throw() should be rejected in general. Rather than
about lambdas here since we don't even warn/error out about `void f() throw()
{}`.

Note `throw()` was removed with 
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0619r4.html