libstdc++: Fix views::transform(move_only_fn{}) forwarding [PR118413]

The range adaptor perfect forwarding simplification mechanism is currently
only enabled for trivially copyable bound arguments, to prevent undesirable
copies of complex objects.  But "trivially copyable" is the wrong property
to check for here, since a move-only type with a trivial move constructor
is considered trivially copyable, and after P2492R2 we can't assume copy
constructibility of the bound arguments.  This patch makes the mechanism
more specifically check for trivial copy constructibility instead so
that it's properly disabled for move-only bound arguments.

	PR libstdc++/118413

libstdc++-v3/ChangeLog:

	* include/std/ranges (views::__adaptor::_Partial): Adjust
	constraints on the "simple" partial specializations to require
	is_trivially_copy_constructible_v instead of
	is_trivially_copyable_v.
	* testsuite/std/ranges/adaptors/adjacent_transform/1.cc (test04):
	Extend P2494R2 test.
	* testsuite/std/ranges/adaptors/transform.cc (test09): Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

3 files changed, 5 insertions, 2 deletions

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index ad69a94..5c795a9 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1145,7 +1145,7 @@ namespace views::__adaptor
   // which makes overload resolution failure diagnostics more concise.
   template<typename _Adaptor, typename... _Args>
     requires __adaptor_has_simple_extra_args<_Adaptor, _Args...>
-      && (is_trivially_copyable_v<_Args> && ...)
+      && (is_trivially_copy_constructible_v<_Args> && ...)
     struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
     {
       tuple<_Args...> _M_args;
@@ -1176,7 +1176,7 @@ namespace views::__adaptor
   // where _Adaptor accepts a single extra argument.
   template<typename _Adaptor, typename _Arg>
     requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
-      && is_trivially_copyable_v<_Arg>
+      && is_trivially_copy_constructible_v<_Arg>
     struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
     {
       _Arg _M_arg;
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc
index a5791b3..772e4b3 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc
@@ -110,6 +110,7 @@ test04()
   };
   // P2494R2 Relaxing range adaptors to allow for move only types
   static_assert( requires { views::pairwise_transform(x, move_only{}); } );
+  static_assert( requires { x | views::pairwise_transform(move_only{}); } );
 }
 
 int
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
index dfc91fb..934d2f6 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
@@ -191,8 +191,10 @@ test09()
 #if __cpp_lib_ranges >= 202207L
   // P2494R2 Relaxing range adaptors to allow for move only types
   static_assert( requires { transform(x, move_only{}); } );
+  static_assert( requires { x | transform(move_only{}); } ); // PR libstdc++/118413
 #else
   static_assert( ! requires { transform(x, move_only{}); } );
+  static_assert( ! requires { x | transform(move_only{}); } );
 #endif
 }