[cfe-dev] __cpp_lib library feature test macros

[Seth Cantrell]

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 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4030.htm 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

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