[Bug libstdc++/104602] New: std::source_location::current uses cast from void*

["foom at fuhm dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104602

            Bug ID: 104602
           Summary: std::source_location::current uses cast from void*
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foom at fuhm dot net
  Target Milestone: ---

I'm working on implementing __builtin_source_location() in Clang
(https://reviews.llvm.org/D120159).

In testing it against the libstdc++ <source_location> header, I ran into a
minor issue.

"current()" in GNU libstdc++ is defined as so:

    static consteval source_location
    current(const void* __p = __builtin_source_location()) noexcept
    {
      source_location __ret;
      __ret._M_impl = static_cast <const __impl*>(__p);
      return __ret;
    }

But! A static_cast from a `const void*` parameter to `const __impl*` is not
permitted in constexpr evaluation:
"""
5. An expression E is a core constant expression unless the evaluation of E,
[...] would evaluate one of the following:
[...]
5.15. a conversion from type cv void* to a pointer-to-object type;"
"""
http://eel.is/c++draft/expr.const#5.15

Clang diagnoses this rule, but GCC apparently does not. (it's not really clear
to me why this rule really needs to exist in the standard -- why bother to
police which kinds of pointer casts you're allowed to do, instead of just
raising an error upon _access_ through the wrong type?)

Anyhow, to workaround this issue, I plan to simply hardcode an exception to the
check in Clang for casts which occur in a "std::source_location::current"
method. Yet, although it's perhaps too late to avoid this workaround, it'd be
nice if libstdc++ didn't require the use of an invalid cast.

In clang (in my proposed change), __builtin_source_location already returns the
expected `const __impl*` type, rather than `const void*` as it does in GCC. So,
the issue is only the cast TO `void*` and back again in libstdc++. ISTM this
would be fixed by moving the `static_cast <const __impl*>` into the default
parameter expression. That would then be a no-op cast on clang, and an (invalid
but undiagnosed) cast from void in GCC.