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

[Marek Polacek <polacek@redhat.com>]

On Fri, Sep 23, 2022 at 03:40:23PM +0100, Jonathan Wakely wrote:
> 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.

Ah, you're right, it must be this lib/Parse/ParseDeclCXX.cpp code:

    // GNU libstdc++ 4.2 and libc++ use certain intrinsic names as the
    // name of struct templates, but some are keywords in GCC >= 4.3
    // and Clang. Therefore, when we see the token sequence "struct
    // X", make X into a normal identifier rather than a keyword, to
    // allow libstdc++ 4.2 and libc++ to work properly.
    TryKeywordIdentFallback(true);
 
Whether we want to do this, I'm not sure.

Marek