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

[liam at liampwll dot com via Gcc-bugs]

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

--- Comment #10 from Liam Powell <liam at liampwll dot com> ---
Lifting the parent of the mutually tagged type out to a non-generic package
works as a workaround. I'm not sure if this can be done internally without a
whole new implementation. This however introduces another error that seems to
be unrelated:

pragma Ada_2022;
pragma Extensions_Allowed (All_Extensions);

package Controller is
   type Mutably_Tagged_Base is tagged null record with Size'Class => 128;

   generic
   package Controller_Inner is
      generic
      package Config is
         type Mutably_Tagged_Child is new Mutably_Tagged_Base with null record;
      end Config;

      package My_Config is new Config;

      generic
         type T is private;
         X : T; --  remove for working workaround
      package G is
      end G;

      package My_G is new G
        (My_Config.Mutably_Tagged_Child'Class,
         My_Config.Mutably_Tagged_Child'(null record) --  remove for working
workaround
        );
   end Controller_Inner;
end Controller;