[gcc r14-11453] libstdc++: Allow std::ranges::to to create unions

[Jonathan Wakely via Gcc-cvs]

https://gcc.gnu.org/g:5b4e362239131967698328f3648efd7b9eea862e

commit r14-11453-g5b4e362239131967698328f3648efd7b9eea862e
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Mar 25 00:27:52 2025 +0000

    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 <tkami...@redhat.com>
    (cherry picked from commit 479a0a8644e5efa0470f032be21ca7749968af42)

Diff:
---
 libstdc++-v3/include/std/ranges                   |  4 ++--
 libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index ede042c54fc4..fdfecf3527fe 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -9388,7 +9388,7 @@ 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>)
        {
@@ -9547,7 +9547,7 @@ 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 000000000000..780ed1fd932b
--- /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);
+}