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Re: [wg14/wg21 liaison] How to handle conflicting 'unreachable' definitions

From: Ville Voutilainen <ville.voutilainen_at_[hidden]>
Date: Thu, 4 Apr 2024 17:30:26 +0300
On Thu, 4 Apr 2024 at 16:15, Aaron Ballman via Liaison
<liaison_at_[hidden]> wrote:
> C23 adds a macro named `unreachable` to <stddef.h>. C++23 adds a
> function named `std::unreachable` to <utility>. What should the
> behavior of this code be when compiled in C++23?
> #include <stddef.h>
> #ifndef unreachable
> #error "oh no, my macro!"
> #endif
> [support.c.headers.general]p1
> (https://eel.is/c++draft/support.c.headers#general-1) says:
> For compatibility with the C standard library, the C++ standard
> library provides the C headers shown in Table 40. The intended use of
> these headers is for interoperability only. It is possible that C++
> source files need to include one of these headers in order to be valid
> ISO C. Source files that are not intended to also be valid ISO C
> should not use any of the C headers.
> I read this as saying that including a C header in a C++
> implementation should provide all the interfaces from the C header
> unless the C++ standard says something else. Since C++23 isn't based
> on top of C23 and thus says nothing about the `unreachable` macro in
> stddef.h, I think this implies that the code should compile.
> (Similarly, including stdbit.h should be fine if the header exists, or
> including string.h and using strdup or memccpy should work, and so on
> for all the newly introduced C23 library functionality.) I think this
> matches user expectations and the intent of the standard, but the
> situation with unreachable is a bit odd and the question came up
> during a code review for Clang which removes the macro from stddef.h
> in C++ mode because of the potential for conflict with the function
> declared in <utility>.
> I have two concrete questions:
> 1) Before WG21 has rebased on top of the latest C, are C++
> implementations expected to strip out all interfaces from C standard
> library headers that are not in the version of C which C++ has based
> the library on?
> 2) If the answer to #1 is "no, C++ implementations are expected to
> provide the C interfaces present unless otherwise stated", is
> unreachable likely to be special when C++ rebases on top of C23?

My spec-squinting suggests that the answer to (1) is yes. The
normative reference to C
comes with a date, it refers to C18. I therefore would not expect
C++23 to bring in C23 entities.

That is also my technical preference; it's ostensibly so that that
particular version of C
has been figured out how to make it work with that particular version
of C++, and newer
versions of C simply might not work.

Why, for heaven's sake, is 'unreachable' a macro in C? That is instant
trouble for C++,
where every user expects to be able to scope names via namespaces.

Received on 2024-04-04 14:30:44