__cpp_lib library feature test macros

Clang Frontend
1

clang implements feature testing macros for language features but I notice that libc++ does not implement the library testing macros. Is this deliberate or would patches to fix this be welcome?

Attached is an initial attempt at implementing Feature-testing recommendations for C++ based on http://libcxx.llvm.org/cxx1y_status.html. No tests are included yet.

Also, about the language feature testing, I notice that they don't take into account features being disabled via -Werror=. It seems like this might matter in cases such as a third party library's headers doing feature testing, but where the local build environment has disabled features for whatever reason. It seems reasonable that a header testing #if __has_extension(__cxx_constexpr__) or #ifdef __cpp_constexpr would use whatever fallback code it's got when the user disables constexpr. Feature testing does take into account features disabled with -fno- arguments.

- Seth

0001-implement-library-feature-test-macros-n4030.patch (8.37 KB)

2

clang implements feature testing macros for language features but I notice
that libc++ does not implement the library testing macros. Is this
deliberate or would patches to fix this be welcome?

Attached is an initial attempt at implementing
Feature-testing recommendations for C++ based
on http://libcxx.llvm.org/cxx1y_status.html. No tests are included yet.

Also, about the language feature testing, I notice that they don't take
into account features being disabled via -Werror=.

Which, if any, features can we disable via -Werror? I don't think we have a
warning for constexpr, for example - we have a generic warning for all
C++11 features (so you can build a codebase as C++11 without allowing the
codebase to become C++98-incompatible), so arguably all the language
features could be "not available" when built with that warning and -Werror,
but that would change the behavior of your program, which would be
problematic if you wanted to make sure that when you build as C++11 the
project works (because then if you disabled the warning, you'd end up with
different behavior - so your experimental results wouldn't be
representative of the end state you were aiming for)

3

Hi All,

CC Marshall,

I’ve had some offline discussions with Marshall about adding feature testing macros to libc++ and there was some hesitation about doing it.
Patches (with test cases) are welcome, but I don’t know if we have decided on a direction yet. There is a similar bug report that can be found here: http://llvm.org/bugs/show_bug.cgi?id=18584.

/Eric