libstdc++: Fix visit<void>(v) for non-void visitors [PR106589]

The optimization for the common case of std::visit forgot to handle the
edge case of passing zero variants to a non-void visitor and converting
the result to void.

libstdc++-v3/ChangeLog:

	PR libstdc++/106589
	* include/std/variant (__do_visit): Handle is_void<R> for zero
	argument case.
	* testsuite/20_util/variant/visit_r.cc: Check std::visit<void>(v).

1 files changed, 8 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/20_util/variant/visit_r.cc b/libstdc++-v3/testsuite/20_util/variant/visit_r.cc
index 712459f..c77b259 100644
--- a/libstdc++-v3/testsuite/20_util/variant/visit_r.cc
+++ b/libstdc++-v3/testsuite/20_util/variant/visit_r.cc
@@ -54,10 +54,18 @@ void test02()
   std::visit<const void>(Visitor(), v);
 }
 
+void test03()
+{
+  // PR libstdc++/106589 - visit<void> rejects lambdas that do not return void
+  auto visitor = []{ return 0; };
+  std::visit<void>(visitor);
+  std::visit<void>(static_cast<int(*)()>(visitor));
+}
 
 int
 main()
 {
   test01();
   test02();
+  test03();
 }