Libc++'s policy is to support only the latest released Xcode, which is
Xcode 16.x. We did update our CI jobs to Xcode 16.x, but we forgot to
update the documentation, which still mentioned Xcode 15. This patch
updates the documentation and cleans up outdated mentions of
apple-clang-15 in the test suite.
We'e specialized `std::signbit` for signed and unsigned integral types
seperately, even though the optimizer can trivially figure out that
`unsigned_value < 0` always false is. This patch removes the
specialization, since there is really not much of a benefit to it.
This patch implements the forwarding to frozen C++03 headers as
discussed in
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc. In the
RFC, we initially proposed selecting the right headers from the Clang
driver, however consensus seemed to steer towards handling this in the
library itself. This patch implements that direction.
At a high level, the changes basically amount to making each public
header look like this:
```
// inside <vector>
#ifdef _LIBCPP_CXX03_LANG
# include <__cxx03/vector>
#else
// normal <vector> content
#endif
```
In most cases, public headers are simple umbrella headers so there isn't
much code in the #else branch. In other cases, the #else branch contains
the actual implementation of the header.
## Why
Following up on https://github.com/llvm/llvm-project/pull/105946, this
patch provides the floating point overloads for `std::signbit` as
defined by
[P0533R9](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0533r9.pdf).
## What
* Test and add overloads for cv-unqualified floating point types
* Remove constrained overload as it is not needed anymore
* Make use of `template<class = void>` as the universal C runtime (UCRT)
needed for Clang-Cl comes with overloads for all cv-unqualified floating
point types (float, double, long double) for `std::signbit()` by itself
[in the
WinSDK](e012b29924/generation/WinSDK/RecompiledIdlHeaders/ucrt/corecrt_math.h (L309-L322)).
In a certain way, this can be seen as a deviation from the C standard.
We need to work around it as the compilation would otherwise error out
due to duplicated definitions.
## Why
Since 18th of August, the floating point comparison builtin
``__builtin_signbit`` is available in Clang as constant expression
(https://github.com/llvm/llvm-project/pull/94118).
## What
* Implement `constexpr` for `std::signbit()` as defined by
[P0533R9](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0533r9.pdf)
(new C++23 feature)
* Restrict execution of tests to tip-of-trunk Clang as builtin is not
yet available (note that builtin is available in GCC)
