[libcxx-commits] [PATCH] D137502: [libc++][math.h] move #undefs to the top and guard explicitly against MSVCRT instead

[Louis Dionne via Phabricator via libcxx-commits]

ldionne added a comment.

In D137502#3941596 <https://reviews.llvm.org/D137502#3941596>, @EricWF wrote:

> This changes behavior.
>
> Currently libc++ only provides its own definitions of a math function if there's a macro which is normally used to define it.
> Now libc++ always provides those overloads. That could cause redefinition errors.
>
> I know there's been a lot of churn in this header recently , so maybe the diff I'm reading is out of date, but please verify that's the case.

Let's talk about `signbit` just to simplify things, but this applies to all of them AFAICT. My understanding is that C implementations are required to make this a macro: https://en.cppreference.com/w/c/numeric/math/signbit

If that were not the case, previously we would not have been defining `signbit`, so we would have been non-confirming because C's `signbit` only works with floating point, whereas C++'s `signbit` is supposed to work with integral types as well: https://en.cppreference.com/w/cpp/numeric/math/signbit

In other words, I'm not sure there are many C implementations that define them as functions, but I may be wrong. I know GCC and the macOS SDK both define them as macros.


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https://reviews.llvm.org/D137502