libstdc++: Fix std::move_only_function for incomplete parameter types

The std::move_only_function::__param_t alias template attempts to
optimize argument passing for the invoker, by passing by rvalue
reference for types that are non-trivial or large. However, the
precondition for is_trivally_copyable makes it unsuitable for using
here, and can cause ODR violations. Just use is_scalar instead, and pass
all class types (even small, trivial ones) by value.

libstdc++-v3/ChangeLog:

	* include/bits/mofunc_impl.h (move_only_function::__param_t):
	Use __is_scalar instead of is_trivially_copyable.
	* testsuite/20_util/move_only_function/call.cc: Check parameters
	involving incomplete types.

2 files changed, 12 insertions, 4 deletions

diff --git a/libstdc++-v3/include/bits/mofunc_impl.h b/libstdc++-v3/include/bits/mofunc_impl.h
index 405c405..47e1e50 100644
--- a/libstdc++-v3/include/bits/mofunc_impl.h
+++ b/libstdc++-v3/include/bits/mofunc_impl.h
@@ -205,10 +205,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     private:
       template<typename _Tp>
-	using __param_t
-	  = __conditional_t<is_trivially_copyable_v<_Tp>
-			      && sizeof(_Tp) <= sizeof(long),
-			    _Tp, _Tp&&>;
+	using __param_t = __conditional_t<is_scalar_v<_Tp>, _Tp, _Tp&&>;
 
       using _Invoker = _Res (*)(_Mofunc_base _GLIBCXX_MOF_CV*,
 				__param_t<_ArgTypes>...) noexcept(_Noex);
diff --git a/libstdc++-v3/testsuite/20_util/move_only_function/call.cc b/libstdc++-v3/testsuite/20_util/move_only_function/call.cc
index 68aa205..3e15983 100644
--- a/libstdc++-v3/testsuite/20_util/move_only_function/call.cc
+++ b/libstdc++-v3/testsuite/20_util/move_only_function/call.cc
@@ -191,10 +191,21 @@ test04()
   VERIFY( std::move(std::as_const(f5))() == 3 );
 }
 
+struct Incomplete;
+
+void
+test_params()
+{
+  std::move_only_function<void(Incomplete)> f1;
+  std::move_only_function<void(Incomplete&)> f2;
+  std::move_only_function<void(Incomplete&&)> f3;
+}
+
 int main()
 {
   test01();
   test02();
   test03();
   test04();
+  test_params();
 }