[gcc r15-7764] libstdc++: Add static_assertions to ranges::to adaptor factory [PR112803]

[Jonathan Wakely via Libstdc++-cvs]

https://gcc.gnu.org/g:a9cfcd0d9e3780c71442057f636f62a7142056cb

commit r15-7764-ga9cfcd0d9e3780c71442057f636f62a7142056cb
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Feb 27 15:48:49 2025 +0000

    libstdc++: Add static_assertions to ranges::to adaptor factory [PR112803]
    
    The standard requires that we reject attempts to create a ranges::to
    adaptor for cv-qualified types and non-class types. Currently we only
    diagnose it once the adaptor is used in a pipeline.
    
    This adds static assertions to diagnose it immediately.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/112803
            * include/std/ranges (ranges::to): Add static assertions to
            enforce Mandates conditions.
            * testsuite/std/ranges/conv/112803.cc: New test.

Diff:
---
 libstdc++-v3/include/std/ranges                  |  3 +++
 libstdc++-v3/testsuite/std/ranges/conv/112803.cc | 20 ++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 6c65722b687e..c0b1134ab328 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -9569,6 +9569,9 @@ namespace __detail
     constexpr auto
     to [[nodiscard]] (_Args&&... __args)
     {
+      static_assert(!is_const_v<_Cont> && !is_volatile_v<_Cont>);
+      static_assert(is_class_v<_Cont>);
+
       using __detail::_To;
       using views::__adaptor::_Partial;
       return _Partial<_To<_Cont>, decay_t<_Args>...>{0, 
std::forward<_Args>(__args)...};
diff --git a/libstdc++-v3/testsuite/std/ranges/conv/112803.cc 
b/libstdc++-v3/testsuite/std/ranges/conv/112803.cc
new file mode 100644
index 000000000000..0a73b0200b0c
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/conv/112803.cc
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++23 } }
+
+// Bug 112803 - <ranges>: to(Args&&... args) is missing Mandates
+
+#include <ranges>
+
+void
+test()
+{
+  struct C { };
+
+  (void) std::ranges::to<int>(); // { dg-error "here" }
+  (void) std::ranges::to<C*>(); // { dg-error "here" }
+  (void) std::ranges::to<C&>(); // { dg-error "here" }
+  (void) std::ranges::to<const C>(); // { dg-error "here" }
+  (void) std::ranges::to<volatile C>(); // { dg-error "here" }
+  (void) std::ranges::to<const volatile C>(); // { dg-error "here" }
+}
+
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }