[libc++] Fix std::is_array<T[0]> and add tests

[ldionne]

Actually fix libc++'s implementation of std::is_array

[ldionne]

Try to workaround clang-cl issue with mangled names by using int[0] instead of char[0]

libcxx/include/type_traits

 
 #if _LIBCPP_STD_VER > 14
 template <class _Tp>
 inline constexpr bool is_floating_point_v = is_floating_point<_Tp>::value;
 #endif
 
 // is_array
 
-#if __has_keyword(__is_array)
+// TODO: Clang incorrectly reports that __is_array is true for T[0].
+//       Re-enable the branch once https://llvm.org/PR54705 is fixed.
+#if __has_keyword(__is_array) && 0
 
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS is_array : _BoolConstant<__is_array(_Tp)> { };
 
 #if _LIBCPP_STD_VER > 14
 template <class _Tp>
 inline constexpr bool is_array_v = __is_array(_Tp);
 #endif

libcxx/test/std/utilities/meta/meta.unary/meta.unary.cat/is_array.pass.cpp

 enum Enum {zero, one};
 struct incomplete_type;
 
 typedef void (*FunctionPtr)();
 
 int main(int, char**)
 {
     test_is_array<char[3]>();
+    test_is_not_array<char[0]>();
     test_is_array<char[]>();
     test_is_array<Union[]>();
 
     test_is_not_array<std::nullptr_t>();
     test_is_not_array<void>();
     test_is_not_array<int&>();
     test_is_not_array<int&&>();
     test_is_not_array<int*>();

libcxx/test/std/utilities/meta/meta.unary/meta.unary.comp/is_bounded_array.pass.cpp

 void test_array()
 {
     test_array_imp<T, B>();
     test_array_imp<const T, B>();
     test_array_imp<volatile T, B>();
     test_array_imp<const volatile T, B>();
 }
 
-typedef char array[3];
-typedef char incomplete_array[];
-
 class incomplete_type;
 
 class Empty {};
 union Union {};
 
 class Abstract
 {
     virtual ~Abstract() = 0;
//  Non-array types
   test_array<int &&,         false>();
     test_array<Empty,          false>();
     test_array<Union,          false>();
     test_array<Abstract,       false>();
     test_array<Enum,           false>();
     test_array<FunctionPtr,    false>();
 
 //  Array types
-    test_array<array,             true>();
-    test_array<incomplete_array,  false>();
+    test_array<char[3],           true>();
+    test_array<int[0],            false>();
+    test_array<char[],            false>();
     test_array<incomplete_type[], false>();
 
   return 0;
 }

libcxx/test/std/utilities/meta/meta.unary/meta.unary.comp/is_unbounded_array.pass.cpp

 void test_array()
 {
     test_array_imp<T, B>();
     test_array_imp<const T, B>();
     test_array_imp<volatile T, B>();
     test_array_imp<const volatile T, B>();
 }
 
-typedef char array[3];
-typedef char incomplete_array[];
-
 class incomplete_type;
 
 class Empty {};
 union Union {};
 
 class Abstract
 {
     virtual ~Abstract() = 0;
//  Non-array types
     test_array<int &&,         false>();
     test_array<Empty,          false>();
     test_array<Union,          false>();
     test_array<Abstract,       false>();
     test_array<Enum,           false>();
     test_array<FunctionPtr,    false>();
 
 //  Array types
-    test_array<array,             false>();
-    test_array<incomplete_array,  true>();
+    test_array<char[3],           false>();
+    test_array<int[0],            false>();
+    test_array<char[],            true>();
     test_array<incomplete_type[], true>();
 
   return 0;
 }