[Bug c++/107126] -Wc++20-compat should complain about template-id in out of class definition for destructor

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107126

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |jason at gcc dot gnu.org
            Summary|GCC accepts invalid out of  |-Wc++20-compat should
                   |class definition for        |complain about template-id
                   |destructor with C++17       |in out of class definition
                   |                            |for destructor
   Last reconfirmed|                            |2022-10-03

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> ---
As Jonathan documents, this was explicitly well-formed from DR1435 in C++11
until P1787 in C++23.  It seems reasonable to also apply the P1787 change to
C++20, treating it as a missing piece of the DR2237 resolution.  It seems
undesirable to apply it to C++17 as well.

But it would be helpful for -Wc++20-compat to flag this compatibility issue

Incidentally, we should probably also reduce the C++20 error to a pedwarn.