[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).

(cherry picked from commit 6a7f0551178e966a686dd48dfa2ea045a35addef)

[ldionne]

This is for cherry-picking onto release/14.x. I'll land this if CI is green.

[jloser]

LGTM! I saw this coming. :)

[ldionne]

This landed a while back in 3f43d803382d57e3fc010ca19833077d1023e9c9.

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_CONCEPTS) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+#if defined(_LIBCPP_HAS_NO_CONCEPTS) || 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

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: libcpp-no-concepts
 
 // <span>
 
 //   template<class It, class EndOrSize>
 //     span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
 //
 //   template<class T, size_t N>
 //     span(T (&)[N]) -> span<T, N>;
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 _LIBCPP_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 // _LIBCPP_HAS_NO_INCOMPLETE_RANGES
 
 int main(int, char**)
 {
   test_iterator_sentinel();
   test_c_array();
   test_std_array();
-
-#ifndef _LIBCPP_HAS_NO_INCOMPLETE_RANGES
   test_range_std_container();
-#endif // _LIBCPP_HAS_NO_INCOMPLETE_RANGES
 
   return 0;
 }