[Bug c++/125563] GCC accepts a default argument that invokes a deleted constructor in the inner class of a class template

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125563

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
I think this is a QoI issue.
The default argument is not used until later on. There might be a full
duplicates of this too.