Re: [PATCH] c++: Implement __is_{nothrow_,}convertible [PR106784]

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 22 Sept 2022 at 23:14, Jason Merrill wrote:
> On 9/22/22 09:39, Marek Polacek wrote:
> > This patch doesn't make libstdc++ use the new built-ins, but I had to
> > rename a class otherwise its name would clash with the new built-in.
>
> Sigh, that's going to be a hassle when comparing compiler versions on
> preprocessed code.

Good point. Clang has some gross hacks that we could consider. When it
sees a declaration that uses a built-in name, it disables the built-in
for the remainder of the translation unit. It does this precisely to
allow a new Clang to compile old std::lib headers where a built-in
like __is_assignable was used as a normal class template, not the
built-in (because no such built-in existed at the time the library
code was written). For us, this is only really a problem when
bisecting bugs and using a newer compiler to compile .ii files from
odler headers, but for Clang combining a new Clang with older
libstdc++ headers is a hard requirement (recall that when Clang was
first deployed to macOS it had to consume the system's libstdc++ 4.2
headers).

It's a big kluge, but it would mean that a new GCC could happily
consume preprocessed code from older libstdc++ headers.