[libc++][math] Fix acceptance of convertible types in std::isnan() and std::isinf() (#98952)

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

libcxx/include/__math/traits.h

@@ -79,20 +79,22 @@ _LIBCPP_NODISCARD _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool isinf
   return false;
 }
 
-#ifdef _LIBCPP_PREFERRED_OVERLOAD
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool isinf(float __x) _NOEXCEPT {
   return __builtin_isinf(__x);
 }
 
-_LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD bool
-isinf(double __x) _NOEXCEPT {
+_LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI
+#ifdef _LIBCPP_PREFERRED_OVERLOAD
+_LIBCPP_PREFERRED_OVERLOAD
+#endif
+    bool
+    isinf(double __x) _NOEXCEPT {
   return __builtin_isinf(__x);
 }
 
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool isinf(long double __x) _NOEXCEPT {
   return __builtin_isinf(__x);
 }
-#endif
 
 // isnan
 
@@ -106,20 +108,22 @@ _LIBCPP_NODISCARD _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool isnan
   return false;
 }
 
-#ifdef _LIBCPP_PREFERRED_OVERLOAD
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool isnan(float __x) _NOEXCEPT {
   return __builtin_isnan(__x);
 }
 
-_LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD bool
-isnan(double __x) _NOEXCEPT {
+_LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI
+#ifdef _LIBCPP_PREFERRED_OVERLOAD
+_LIBCPP_PREFERRED_OVERLOAD
+#endif
+    bool
+    isnan(double __x) _NOEXCEPT {
   return __builtin_isnan(__x);
 }
 
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool isnan(long double __x) _NOEXCEPT {
   return __builtin_isnan(__x);
 }
-#endif
 
 // isnormal
 

libcxx/test/std/numerics/c.math/cmath.pass.cpp

@@ -708,15 +708,16 @@ void test_isinf()
     static_assert((std::is_same<decltype(std::isinf((float)0)), bool>::value), "");
 
     typedef decltype(std::isinf((double)0)) DoubleRetType;
-#if !defined(__linux__) || defined(__clang__)
-    static_assert((std::is_same<DoubleRetType, bool>::value), "");
-#else
+#if defined(__GLIBC__) && TEST_STD_VER == 03 && defined(TEST_COMPILER_CLANG)
     // GLIBC < 2.23 defines 'isinf(double)' with a return type of 'int' in
     // all C++ dialects. The test should tolerate this when libc++ can't work
-    // around it.
+    // around it via `_LIBCPP_PREFERRED_OVERLOAD`, which is only available
+    // in modern versions of Clang, and not elsewhere.
     // See: https://sourceware.org/bugzilla/show_bug.cgi?id=19439
     static_assert((std::is_same<DoubleRetType, bool>::value
                 || std::is_same<DoubleRetType, int>::value), "");
+#else
+    static_assert((std::is_same<DoubleRetType, bool>::value), "");
 #endif
 
     static_assert((std::is_same<decltype(std::isinf(0)), bool>::value), "");
@@ -794,15 +795,16 @@ void test_isnan()
     static_assert((std::is_same<decltype(std::isnan((float)0)), bool>::value), "");
 
     typedef decltype(std::isnan((double)0)) DoubleRetType;
-#if !defined(__linux__) || defined(__clang__)
-    static_assert((std::is_same<DoubleRetType, bool>::value), "");
-#else
-    // GLIBC < 2.23 defines 'isinf(double)' with a return type of 'int' in
+#if defined(__GLIBC__) && TEST_STD_VER == 03 && defined(TEST_COMPILER_CLANG)
+    // GLIBC < 2.23 defines 'isnan(double)' with a return type of 'int' in
     // all C++ dialects. The test should tolerate this when libc++ can't work
-    // around it.
+    // around it via `_LIBCPP_PREFERRED_OVERLOAD`, which is only available
+    // in modern versions of Clang, and not elsewhere.
     // See: https://sourceware.org/bugzilla/show_bug.cgi?id=19439
     static_assert((std::is_same<DoubleRetType, bool>::value
                 || std::is_same<DoubleRetType, int>::value), "");
+#else
+    static_assert((std::is_same<DoubleRetType, bool>::value), "");
 #endif
 
     static_assert((std::is_same<decltype(std::isnan(0)), bool>::value), "");

libcxx/test/std/numerics/c.math/isinf.pass.cpp

@@ -62,9 +62,21 @@ struct TestInt {
   }
 };
 
+template <typename T>
+struct ConvertibleTo {
+  operator T() const { return T(); }
+};
+
 int main(int, char**) {
   types::for_each(types::floating_point_types(), TestFloat());
   types::for_each(types::integral_types(), TestInt());
 
+  // Make sure we can call `std::isinf` with convertible types
+  {
+    assert(!std::isinf(ConvertibleTo<float>()));
+    assert(!std::isinf(ConvertibleTo<double>()));
+    assert(!std::isinf(ConvertibleTo<long double>()));
+  }
+
   return 0;
 }

libcxx/test/std/numerics/c.math/isnan.pass.cpp

@@ -62,9 +62,21 @@ struct TestInt {
   }
 };
 
+template <typename T>
+struct ConvertibleTo {
+  operator T() const { return T(); }
+};
+
 int main(int, char**) {
   types::for_each(types::floating_point_types(), TestFloat());
   types::for_each(types::integral_types(), TestInt());
 
+  // Make sure we can call `std::isnan` with convertible types
+  {
+    assert(!std::isnan(ConvertibleTo<float>()));
+    assert(!std::isnan(ConvertibleTo<double>()));
+    assert(!std::isnan(ConvertibleTo<long double>()));
+  }
+
   return 0;
 }