[Bug ada/123306] assertion failure for unfrozen mutably tagged type as actual in instance

[liam at liampwll dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123306

--- Comment #5 from Liam Powell <liam at liampwll dot com> ---
Thanks.

Workaround in case anyone else comes across this before 15.3: Just force the
type to be frozen by declaring a constant of the non-classwide type.

Interestingly `X : constant Mutably_Tagged;` does not work for this but `X :
constant Mutably_Tagged := (null record);` does as well as `X :
Mutably_Tagged;` (without constant).