[libc++] Re-enable workaround for pre-ranges CTAD in std::span

[ldionne]

Summary

See https://reviews.llvm.org/D121626 for details -- this re-enables the
CTAD we removed, since it does break some stuff as well (even though it's
not nearly as bad as the removed constructors fixed by D121626).

[ldionne]

Just running CI, I'll ship this if green since this is a small follow-up to D121626.

libcxx/include/span

template<class _Tp, size_t _Sz>
     span(_Tp (&)[_Sz]) -> span<_Tp, _Sz>;
 
 template<class _Tp, size_t _Sz>
     span(array<_Tp, _Sz>&) -> span<_Tp, _Sz>;
 
 template<class _Tp, size_t _Sz>
     span(const array<_Tp, _Sz>&) -> span<const _Tp, _Sz>;
 
-#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#if defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+template<class _Container>
+    span(_Container&) -> span<typename _Container::value_type>;
+
+template<class _Container>
+    span(const _Container&) -> span<const typename _Container::value_type>;
+#else
 template<ranges::contiguous_range _Range>
     span(_Range&&) -> span<remove_reference_t<ranges::range_reference_t<_Range>>>;
 #endif
 
 #endif // _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_SPAN

libcxx/test/std/containers/views/span.cons/deduct.pass.cpp

void test_std_array() {
     const std::array<long, 5> arr = {4, 5, 6, 7, 8};
     std::span s{arr};
     ASSERT_SAME_TYPE(decltype(s), std::span<const long, 5>);
     assert(s.size() == arr.size());
     assert(s.data() == arr.data());
     }
 }
 
-#ifndef TEST_HAS_NO_INCOMPLETE_RANGES
 void test_range_std_container() {
     {
     std::string str{"ABCDE"};
     std::span s{str};
     ASSERT_SAME_TYPE(decltype(s), std::span<char>);
     assert(s.size() == str.size());
     assert(s.data() == str.data());
     }
 
     {
     const std::string str{"QWERTYUIOP"};
     std::span s{str};
     ASSERT_SAME_TYPE(decltype(s), std::span<const char>);
     assert(s.size() == str.size());
     assert(s.data() == str.data());
     }
 }
-#endif // TEST_HAS_NO_INCOMPLETE_RANGES
 
 int main(int, char**)
 {
   test_iterator_sentinel();
   test_c_array();
   test_std_array();
-
-#ifndef TEST_HAS_NO_INCOMPLETE_RANGES
   test_range_std_container();
-#endif // TEST_HAS_NO_INCOMPLETE_RANGES
 
   return 0;
 }

libcxx/test/support/test_macros.h

 #if defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
 #  define TEST_HAS_NO_FILESYSTEM_LIBRARY
 #endif
 
 #if defined(_LIBCPP_HAS_NO_FGETPOS_FSETPOS)
 #  define TEST_HAS_NO_FGETPOS_FSETPOS
 #endif
 
-#if defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
-#  define TEST_HAS_NO_INCOMPLETE_RANGES
-#endif
-
 #if defined(TEST_COMPILER_CLANG)
 #  define TEST_DIAGNOSTIC_PUSH _Pragma("clang diagnostic push")
 #  define TEST_DIAGNOSTIC_POP _Pragma("clang diagnostic pop")
 #  define TEST_CLANG_DIAGNOSTIC_IGNORED(str) _Pragma(TEST_STRINGIZE(clang diagnostic ignored str))
 #  define TEST_GCC_DIAGNOSTIC_IGNORED(str)
 #  define TEST_MSVC_DIAGNOSTIC_IGNORED(num)
 #elif defined(TEST_COMPILER_GCC)
 #  define TEST_DIAGNOSTIC_PUSH _Pragma("GCC diagnostic push")