From: Jonathan Wakely <jwakely@redhat.com>
Date: Thu, 27 Feb 2025 15:48:49 +0000 (+0000)
Subject: libstdc++: Add static_assertions to ranges::to adaptor factory [PR112803]
X-Git-Tag: releases/gcc-14.3.0~347
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=679af7b1037ae8fd4fe3f7b22ba03673b1af8c98;p=thirdparty%2Fgcc.git

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.

(cherry picked from commit a9cfcd0d9e3780c71442057f636f62a7142056cb)
---

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index c2dcb0265660..ede042c54fc4 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -9546,6 +9546,9 @@ 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 }