[Bug c++/97710] Help with breaking change? cast pointer-to-member address in template parameter list

["mick.pearson at wildblue dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97710

--- Comment #2 from Mick P. <mick.pearson at wildblue dot net> ---
Sorry, I may have jumped to conclusion. I think there's a different problem
that goes like:

struct A{};
struct B:A{ int x; };
struct C:A{ T<A,(char A::*)&B::x> y; };

So T expects a pointer-to-member from A but will not cast (in template?) B's
member to an A type to satisfy the template.

The problem is with C++'s class template model it's very hard to define
pointer-to-member because they don't work like function templates, so you have
to define all of the parts of the pointer-to-member value first and satisfy
those in the parameter list, so it's easier to do casting logic inside the
template instance... and probably in my use case that's the only way to do it
for more technical reasons.

Anyway, this is a breaking-change if it's not understood. It worked in GCC 7
that I upgraded from.

These pointer-to-member values are second-class citizens in C++ since they're
really just simple short offsets (numbers) but they are a royal PITA to deal
with. We could use some tools where the standard comes up short.