[Bug c++/49171] [C++0x][constexpr] Constant expressions support reinterpret_cast

[comexk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49171

--- Comment #24 from comexk at gmail dot com ---
>  All that's needed for that is an extension to provide a compile-time constant value to a pointer, not to allow arbitrary reinterpret_casts in constexpr.

Well, there aren't that many things that reinterpret_cast can do.  Casting
pointers to different pointer types would be useful, but using such pointers is
normally undefined behavior anyway.  The exception is accessing byte
representations using `char *` pointers.  Supporting that in constexpr context
*would* be a major feature, but it doesn't need to be allowed just because the
cast itself is allowed.

On the other hand, C++20 is adding an alternate way to manipulate byte
representations in constexpr context, std::bit_cast.  (It's non-constexpr when
the types involved contain unions or pointers, so it can't be used directly in
place of reinterpret_cast.)  Most of the work that would be needed to support
access-through-`char *` as an extension needs to be done anyway for
std::bit_cast; on the other hand, its existence reduces the need for an
extension.

Regardless, it would be nice to lift the pointer restriction on std::bit_cast
as an extension.