[gcc r14-11259] libstdc++: Fix views::transform(move_only_fn{}) forwarding [PR118413]

[Patrick Palka]

https://gcc.gnu.org/g:f0420cc224db64efe87cbe2f45d4d7ba8deb9eb0

commit r14-11259-gf0420cc224db64efe87cbe2f45d4d7ba8deb9eb0
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jan 29 10:02:28 2025 -0500

    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>
    (cherry picked from commit 09d1cbee10b8c51aed48f047f30717f622d6f811)

Diff:
---
 libstdc++-v3/include/std/ranges                                    | 4 ++--
 libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc | 1 +
 libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc            | 2 ++
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index eba539176848..a966699479bb 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1122,7 +1122,7 @@ 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;
@@ -1153,7 +1153,7 @@ 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 a5791b3da702..772e4b3b6a0d 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 e1b74353e632..783b0fba7e1e 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
 }