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044b892c79
* The only exception is that the flag -std=c++2a is still used not to break compatibility with older compilers (clang <= 9, gcc <= 9). * Bump _LIBCPP_STD_VER for C++20 to 20 and use 21 for the future standard (C++2b). That's a preparation step to add c++2b support to libc++. Reviewed By: ldionne, #libc Differential Revision: https://reviews.llvm.org/D93383
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===================
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Feature Test Macros
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===================
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.. contents::
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:local:
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Overview
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========
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Libc++ implements the C++ feature test macros as specified in the C++20 standard,
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and before that in non-normative guiding documents
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(`See cppreference <https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros>`_)
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Design
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======
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Feature test macros are tricky to track, implement, test, and document correctly.
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They must be available from a list of headers, they may have different values in
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different dialects, and they may or may not be implemented by libc++. In order to
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track all of these conditions correctly and easily, we want a Single Source of
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Truth (SSoT) that defines each feature test macro, its values, the headers it
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lives in, and whether or not is is implemented by libc++. From this SSoA we
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have enough information to automatically generate the `<version>` header,
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the tests, and the documentation.
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Therefore we maintain a SSoA in `libcxx/utils/generate_feature_test_macro_components.py`
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which doubles as a script to generate the following components:
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* The `<version>` header.
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* The version tests under `support.limits.general`.
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* Documentation of libc++'s implementation of each macro.
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Usage
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=====
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The `generate_feature_test_macro_components.py` script is used to track and
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update feature test macros in libc++.
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Whenever a feature test macro is added or changed, the table should be updated
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and the script should be re-ran. The script will clobber the existing test files,
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the documentation and the `<version>` header.
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