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Following up on https://github.com/llvm/llvm-project/pull/98841. Changes: - Properly test convertible types for `std::isnan()` and `std::inf()` - Tighten conditional in `cmath.pass.cpp` (Find insights on `_LIBCPP_PREFERRED_OVERLOAD` below) - Tighten preprocessor guard in `traits.h` Insights into why `_LIBCPP_PREFERRED_OVERLOAD` is needed: (i) When libc++ is layered on top of glibc on Linux, glibc's `math.h` is included. When compiling with `-std=c++03`, this header brings the function declaration of `isinf(double)` [1] and `isnan(double)` [2] into scope. This differs from the C99 Standard as only the macros `#define isnan(arg)` and `#define isinf(arg)` are expected. Therefore, libc++ needs to respect the presense of the `double` overload and cannot redefine it as it will conflict with the declaration already in scope. For `-std=c++11` and beyond this issue is fixed, as glibc guards both the `isinf` and `isnan` by preprocessor macros. (ii) When libc++ is layered on top of Bionic's libc, `math.h` exposes a function prototype for `isinf(double)` with return type `int`. This function prototype in Bionic's libc is not guarded by any preprocessor macros [3]. `_LIBCPP_PREFERRED_OVERLOAD` specifies that a given overload is a better match than an otherwise equally good function declaration. This is implemented in modern versions of Clang via `__attribute__((__enable_if__))`, and not elsewhere. See [4] for details. We use `_LIBCPP_PREFERRED_OVERLOAD` to define overloads in the global namespace that displace the overloads provided by the C libraries mentioned above. [1]:fe94080875/math/bits/mathcalls.h (L185-L194)[2]:fe94080875/math/bits/mathcalls.h (L222-L231)[3]: https://cs.android.com/android/platform/superproject/+/master:bionic/libc/include/math.h;l=322-323;drc=master?hl=fr-BE%22https:%2F%2Fsupport.google.com%2Fmerchants%2Fanswer%2F188494%5C%22%22https:%2F%2Fsupport.google.com%2Fmerchants%2Fanswer%2F188494%5C%22 [4]:5fd17ab1b0
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