[Bug libstdc++/96416] address_of() is broken by static_assert in pointer_traits

["glenjofe at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96416

--- Comment #11 from Glen Joseph Fernandes <glenjofe at gmail dot com> ---
> if it can never be used.

You're misunderstanding.   to_address(p) requires that pointer_traits<P> is
valid. It just doesn't need to have a to_address member function.

Example 1. You have a pointer-like type Ptr1. You haven't specialized
pointer_traits<Ptr1>, but pointer_traits<Ptr1> is valid. Here it will call
to_address(p1.operator->()).

Example 2. You have a pointer-like type Ptr2. You have specialized
pointer_traits<Ptr2> with a to_address function. Here it will call
pointer_traits<Ptr2>::to_address(p2).

to_address() was intended for used with pointers and pointer-like types (and
pointer_traits<Ptr> was always required to be valid).

It was intended for use with allocator pointers, and the original standard
library implementations had a return type of: typename
pointer_traits<Ptr>::element_type*

If (for contiguous iterators, which came later) you want pointer_traits<X> to
be valid even when X does not have element_type, that is a design change to
pointer_traits.