[Bug c++/78495] [7 regression][new inheriting ctors] variant members lead to uninitialized parameter

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78495

--- Comment #2 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
Created attachment 40543
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40543&action=edit
reduced testcase

The problem is not with unnamed union members, but with inheriting ctors
passing by value.  The temporary 'Ptr' object is passed by implicit reference,
but in the inheriting ctor we pass the address of that reference to the Base
ctor.   In this case that means we end up initializing Base::val with a pointer
into the stack frame, rather than NULL.

Note the original testcase has a typo ': val(static_cast<canary&&>(val))' --
the ctor parm and the member variable have the same name, so that's doing self
initialization -- not copying the incoming parm. (not that that;s the bug being
exposed).