[cfe-dev] [RFC] Make <atomic> and <stdatomic.h> explicitly incompatible

[Tim Northover via cfe-dev]

On 12 March 2018 at 21:22, Marcus Johnson via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> I VERY strongly disagree with this idea, this is a horrible idea for compatibility between the languages.
>
> What are you thinking OP? Do you really hate C software that much?

They are already incompatible. This is just making the error message
explain why and happen early.

> My suggestion is to define <atomic> to include <stdatomics.h> and redefine what you need for C++ there.

That's not possible without violating the C++ standard. You can tell
from a C++ program whether atomic_int (for example) is a typedef of
"std::atomic<int>" or "_Atomic(int)", and changing it would break ABI
compatibility. It's simply not on the table.

However, you might be pleased to hear that the C++ committee is
working on making the two properly compatible for a reasonable subset
of uses. That has to be reasonably carefully coordinated to make sure
as little as possible breaks.

The approach is essentially the reverse of what you're proposing: add
a C++-specific stdatomic.h that makes common things like atomic_int,
_Atomic(int) work. Adding it now would be premature, but keep a look
out.

Cheers.

Tim.