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

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 6a7f055)

Differential Revision: https://reviews.llvm.org/D122201

Author: ldionne

libcxx/include/span

@@ -622,7 +622,13 @@ template<class _Tp, size_t _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

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

@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: libcpp-no-concepts
 
 // <span>
 
@@ -86,7 +85,6 @@ void test_std_array() {
     }
 }
 
-#ifndef _LIBCPP_HAS_NO_INCOMPLETE_RANGES
 void test_range_std_container() {
     {
     std::string str{"ABCDE"};
@@ -104,17 +102,13 @@ void test_range_std_container() {
     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;
 }