[libcxx patch] Let module.modulemap require cxx_atomics for the atomics module

[Richard Smith]

On Fri, Sep 19, 2014 at 6:35 PM, Nico Weber <thakis at chromium.org> wrote:

> Hi,
>
> when using modules in c++98, libc++ currently fails to build:
>
> While building module 'std' imported from src/disk_interface.h:18:
> In file included from <module-includes>:3:
> /Users/thakis/src/chrome/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/atomic:539:2:
> error: <atomic> is not implemented
> #error <atomic> is not implemented
>  ^
> In file included from src/disk_interface.cc:15:
> src/disk_interface.h:18:10: fatal error: could not build module 'std'
> #include <map>
>  ~~~~~~~~^
>
> The attached patch fixes this by making atomic depend on cxx_atomics.
> Fixes PR21002.
>

Doesn't this just make the atomic submodule always unavailable? We don't
have a cxx_atomics feature, as far as I can see. And cxx_atomic (the
corresponding __has_feature flag) wouldn't work either: module features are
not the same as __has_feature features (see the relevant documentation
here: http://clang.llvm.org/docs/Modules.html#requires-declaration).
cplusplus11 is the best check we have for this.
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