libstdc++: Non-triv-copyable extra args aren't simple [PR100940]

This force-enables perfect forwarding call wrapper semantics whenever
the extra arguments of a partially applied range adaptor aren't all
trivially copyable, so as to avoid incurring unnecessary copies of
potentially expensive-to-copy objects (such as std::function objects)
when invoking the adaptor.

	PR libstdc++/100940

libstdc++-v3/ChangeLog:

	* include/std/ranges (__adaptor::_Partial): For the "simple"
	forwarding partial specializations, also require that
	the extra arguments are trivially copyable.
	* testsuite/std/ranges/adaptors/100577.cc (test04): New test.

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index a428900..73c35da 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -896,11 +896,12 @@ namespace views::__adaptor
     };
 
   // Partial specialization of the primary template for the case where the extra
-  // arguments of the adaptor can always be safely forwarded by const reference.
-  // This lets us get away with a single operator() overload, which makes
-  // overload resolution failure diagnostics more concise.
+  // arguments of the adaptor can always be safely and efficiently forwarded by
+  // const reference.  This lets us get away with a single operator() overload,
+  // 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> && ...)
     struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure
     {
       tuple<_Args...> _M_args;
@@ -930,6 +931,7 @@ 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>
     struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
     {
       _Arg _M_arg;

diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc
index 8ef0846..06be498 100644 (file)
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc
@@ -21,6 +21,7 @@
 // PR libstdc++/100577
 
 #include <ranges>
+#include <functional>
 
 namespace ranges = std::ranges;
 namespace views = std::ranges::views;
@@ -113,4 +114,17 @@ test03()
   x | std::views::drop(S{});
 }
 
+void
+test04()
+{
+  // Non-trivially-copyable extra arguments make a closure not simple.
+  using F = std::function<bool(bool)>;
+  static_assert(!std::is_trivially_copyable_v<F>);
+  using views::__adaptor::__closure_has_simple_call_op;
+  static_assert(!__closure_has_simple_call_op<decltype(views::take_while(std::declval<F>()))>);
+  static_assert(!__closure_has_simple_call_op<decltype(views::drop_while(std::declval<F>()))>);
+  static_assert(!__closure_has_simple_call_op<decltype(views::filter(std::declval<F>()))>);
+  static_assert(!__closure_has_simple_call_op<decltype(views::transform(std::declval<F>()))>);
+}
+
 // { dg-prune-output "in requirements" }