Implement feature test macros using a script.

Summary:
This patch implements all the feature test macros libc++ currently supports, as specified by the standard or cppreference prior to C++2a.

The tests and `<version>` header are generated using a script. The script contains a table of each feature test macro, the headers it should be accessible from, and its values of each dialect of C++.
When a new feature test macro is added or needed, the table should be updated and the script re-run.



Reviewers: mclow.lists, jfb, serge-sans-paille

Reviewed By: mclow.lists

Subscribers: arphaman, jfb, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D56750

llvm-svn: 351286

1 files changed, 44 insertions, 0 deletions

diff --git a/libcxx/docs/DesignDocs/FeatureTestMacros.rst b/libcxx/docs/DesignDocs/FeatureTestMacros.rst
new file mode 100644
index 00000000000..d55af96c674
--- /dev/null
+++ b/libcxx/docs/DesignDocs/FeatureTestMacros.rst
@@ -0,0 +1,44 @@
+===================
+Feature Test Macros
+===================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+Libc++ implements the C++ feature test macros as specified in the C++2a standard,
+and before that in non-normative guiding documents (`See cppreference <https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros>`)
+
+Design
+======
+
+Feature test macros are tricky to track, implement, test, and document correctly.
+They must be available from a list of headers, they may have different values in
+different dialects, and they may or may not be implemented by libc++. In order to
+track all of these conditions correctly and easily, we want a Single Source of
+Truth (SSoT) that defines each feature test macro, its values, the headers it
+lives in, and whether or not is is implemented by libc++. From this SSoA we
+have enough information to automatically generate the `<version>` header,
+the tests, and the documentation.
+
+Therefore we maintain a SSoA in
+`libcxx/test/std/language.support/support.limits/support.limits.general/generate_feature_test_macro_components.py`
+which doubles as a script to generate the following components:
+
+* The `<version>` header.
+* The version tests under `support.limits.general`.
+* Documentation of libc++'s implementation of each macro.
+
+Usage
+=====
+
+The `generate_feature_test_macro_components.py` script is used to track and
+update feature test macros in libc++.
+
+Whenever a feature test macro is added or changed, the table should be updated
+and the script should be re-ran. The script will clobber the existing test files
+and the documentation and it will generate a new `<version>` header as a
+temporary file. The generated `<version>` header should be merged with the
+existing one.
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