libstdc++: Allow std::ranges::to to create unions

LWG 4229 points out that the std::ranges::to wording refers to class
types, but I added an assertion using std::is_class_v which only allows
non-union class types. LWG consensus is that unions should be allowed,
so this additionally uses std::is_union_v.

libstdc++-v3/ChangeLog:

	* include/std/ranges (ranges::to): Allow unions as well as
	non-union class types.
	* testsuite/std/ranges/conv/lwg4229.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

2 files changed, 20 insertions, 2 deletions

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 34c6f11..7a339c5 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -9421,7 +9421,7 @@ namespace __detail
     to [[nodiscard]] (_Rg&& __r, _Args&&... __args)
     {
       static_assert(!is_const_v<_Cont> && !is_volatile_v<_Cont>);
-      static_assert(is_class_v<_Cont>);
+      static_assert(is_class_v<_Cont> || is_union_v<_Cont>);
 
       if constexpr (__detail::__toable<_Cont, _Rg>)
 	{
@@ -9580,7 +9580,7 @@ namespace __detail
     to [[nodiscard]] (_Args&&... __args)
     {
       static_assert(!is_const_v<_Cont> && !is_volatile_v<_Cont>);
-      static_assert(is_class_v<_Cont>);
+      static_assert(is_class_v<_Cont> || is_union_v<_Cont>);
 
       using __detail::_To;
       using views::__adaptor::_Partial;
diff --git a/libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc b/libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc
new file mode 100644
index 0000000..780ed1f
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++23 } }
+
+// LWG 4229 std::ranges::to with union return type
+
+#include <ranges>
+
+union U
+{
+  template<std::ranges::input_range R> U(std::from_range_t, R&&) { }
+
+  int i;
+};
+
+void
+test_lwg4229(std::ranges::subrange<int*> r)
+{
+  U u = std::ranges::to<U>(r);
+}