[PATCH] PR libstdc++/79433 no #error for including headers with wrong -std

[Jonathan Wakely]

On 07/09/17 15:18 +0100, Jonathan Wakely wrote:
>As discussed in PR 79433, the recommended way to test for new features
>such as std::optional has problems. The current version of SD-6 at
>https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations
>says to simply check __has_include(optional). This test will be true
>even when using -std=c++98, but actually including the header gives an
>error. This works OK for ahead-of-time autoconf-style checks, but not
>for preprocessor-based checks directly in the source code.
>
>The latest draft of SD-6 (to be published soon) changes the
>recommendation to use __has_include, then include the header and also
>check for a feature-test macro e.g.
>
>#ifdef __has_include
># if __has_include(optional)
>#  include <optional>
>#  if __cpp_lib_optional >= 201606
>#   define HAVE_STD_OPTIONAL
>#  endif
># endif
>#endif
>
>For this to work we need to make including <optional> always valid
>(even in C++{98,03,11,14} modes) instead of failing with #error. When
>included pre-C++17 the header should be empty, and specifically not
>define the __cpp_lib_optional macro.
>
>This implements that change. The <shared_mutex> header is also
>affected for C++14, as that defines std::shared_timed_mutex in C++14
>mode, and adds std::shared_mutex in C++17 mode.
>
>With this change nothing else includes c++17_warning.h so we can
>remove it.

I've committed that change to trunk now.

>In a follow-up patch I plan to do the same for the <experimental/*>
>headers. The TS documents already give a macro for every header, and
>LibFundTS suggests testing both __has_include and a macro, see ¶3 at
>https://rawgit.com/cplusplus/fundamentals-ts/v2/fundamentals-ts.html#general.feature.test

And here's the follow-up for the TS headers, also committed to trunk.


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