[diff.library] Entry for `abs` incompatibility

[eisenwave]

The following code has undefined behavior in C++, but is well-defined in C:

abs(LLONG_MIN);

This happens because in C++, abs is overloaded and dispatches to llabs,
whereas in C, LLONG_MIN yields zero when converted to int when calling int abs(int).

[eisenwave]

What's also interesting is that we note in https://eel.is/c++draft/cstdlib.syn#1 that the contents of <cstdlib> and <stdlib.h> are the same "except as noted in [c.math.abs]", and https://eel.is/c++draft/c.math.abs does not say that the additional overloads do not exist in <stdlib.h>.

My reading is that long long abs(long long) is required to exist in both <stdlib.h> and in <cstdlib>, which makes this a genuine incompatibility.

However, neither libstdc++, libc++, nor MSVC STL provide the extra abs overloads in <stdlib.h>, only in <cstdlib>, so what's the intent here?

CC @jwakely

[eisenwave]

I guess LWG3093 is vaguely related.

[frederick-vs-ja]

However, neither libstdc++, libc++, nor MSVC STL provide the extra abs overloads in <stdlib.h>, only in <cstdlib>, so what's the intent here?

MS UCRT provides additional abs and div overloads in <stdlib.h>. Note that MSVC STL doesn't augment C standard library headers itself.

For libstdc++ and libc++, I believe they do provide abs and div overloads in their <stdlib.h>:

• https://github.com/llvm/llvm-project/blob/0285f1c6b791fb974a71a236e329848cf2d2fbe9/libcxx/include/stdlib.h#L109-L110
• https://github.com/gcc-mirror/gcc/blob/a171c89fbf6720bf8562b2e0121e582fbadd5e50/libstdc%2B%2B-v3/include/c_compatibility/stdlib.h#L36

[eisenwave]

Hmm okay, I guess I just got stumped by the fact that Clang seems to provide a non-constexpr, non-C++ std::abs when including <stdlib.h> rather than <cstdlib>, but that may just be a bug (https://godbolt.org/z/vsr3MezW3)