libstdc++: add initializer_list constructor to std::span (P2447R6)

This commit implements P2447R6. The code is straightforward (just one
extra constructor, with constraints and conditional explicit).

I decided to suppress -Winit-list-lifetime because otherwise it would
give too many false positives. The new constructor is meant to be used
as a parameter-passing interface (this is a design choice, see
P2447R6/§2) and, as such, the initializer_list won't dangle despite
GCC's warnings.

The new constructor isn't 100% backwards compatible. A couple of
examples are included in Annex C, but I have also lifted some more
from R4. A new test checks for the old and the new behaviors.

libstdc++-v3/ChangeLog:

	* include/bits/version.def: Add the new feature-testing macro.
	* include/bits/version.h: Regenerate.
	* include/std/span: Add constructor from initializer_list.
	* testsuite/23_containers/span/init_list_cons.cc: New test.
	* testsuite/23_containers/span/init_list_cons_neg.cc: New test.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index 4cee890..4b40bd0 100644
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -39,6 +39,7 @@
 #endif
 
 #define __glibcxx_want_span
+#define __glibcxx_want_span_initializer_list
 #include <bits/version.h>
 
 #ifdef __cpp_lib_span // C++ >= 20 && concepts
@@ -46,6 +47,9 @@
 #include <cstddef>
 #include <bits/stl_iterator.h>
 #include <bits/ranges_base.h>
+#ifdef __cpp_lib_span_initializer_list
+# include <initializer_list>
+#endif
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -228,6 +232,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: _M_ptr(ranges::data(__range)), _M_extent(ranges::size(__range))
 	{ }
 
+#if __cpp_lib_span_initializer_list >= 202311L // >= C++26
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Winit-list-lifetime"
+	constexpr
+	explicit(extent != dynamic_extent)
+	span(initializer_list<value_type> __il)
+	requires (is_const_v<_Type>)
+	: _M_ptr(__il.begin()), _M_extent(__il.size())
+	{
+	}
+#pragma GCC diagnostic pop
+#endif
+
       constexpr
       span(const span&) noexcept = default;