[libcxx] teach type_traits test about long uint32_t

[bcain]

Summary

Patch by Ben Craig.

[EricWF]

This LGTM.

Although wouldn't unsigned int be the correct choice on all platforms? Because the issue only arises because sometimes uint32_t is unsigned long instead of unsigned int?

[bcain]

In D61655#1494372, @EricWF wrote:

This LGTM.

Although wouldn't unsigned int be the correct choice on all platforms? Because the issue only arises because sometimes uint32_t is unsigned long instead of unsigned int?

The standard only guarantees that unsigned int is >= 16 bit so IIUC in that case it can't be compared with char32_t here.

C++17 [basic.fundamental] => C99 5.2.4.2.1

I'm not a language lawyer, I had to look this one up. But AFAICT that's why unsigned int is insufficiently portable for this test.

libcxx/test/libcxx/type_traits/convert_to_integral.pass.cpp

 int main(int, char**)
 {
   check_integral_types<bool, int>();
   check_integral_types<char, int>();
   check_integral_types<signed char, int>();
   check_integral_types<unsigned char, int>();
   check_integral_types<wchar_t, decltype(((wchar_t)1) + 1)>();
   check_integral_types<char16_t, int>();
-  check_integral_types<char32_t, uint32_t>();
+  // On some platforms, unsigned int and long are the same size.  These
+  // platforms have a choice of making uint32_t an int or a long.  However
+  // char32_t must promote to an unsigned int on these platforms [conv.prom].
+  // Use the following logic to make the test work on such platforms.
+  // (sizeof(uint32_t) == sizeof(unsigned int)) ? unsigned int : uint32_t;
+  typedef std::conditional<sizeof(uint32_t) == sizeof(unsigned int),
+                           unsigned int, uint32_t>::type char_integral;
+  check_integral_types<char32_t, char_integral>();
   check_integral_types<short, int>();
   check_integral_types<unsigned short, int>();
   check_integral_types<int, int>();
   check_integral_types<unsigned, unsigned>();
   check_integral_types<long, long>();
   check_integral_types<unsigned long, unsigned long>();
   check_integral_types<long long, long long>();
   check_integral_types<unsigned long long, unsigned long long>();