libstdc++: Fix views::zip_transform constraints for empty range pack [PR111138]

Add missing move_constructible && regular_invocable constrains on functor type,
and is_object on functor result type for invocations of views::zip_transform
without range arguments.

	PR libstdc++/111138

libstdc++-v3/ChangeLog:

	* include/std/ranges (_ZipTransform::operator()):
	Create separate overload for calls with empty range pack,
	and add move_constructible, regular_invocable and
	is_object_v<invoke_result_t<...>>> constraints.
	* testsuite/std/ranges/zip_transform/1.cc: New tests

Reviewed-by: Patrick Palka <ppalka@redhat.com>
	Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/ranges/zip_transform/1.cc b/libstdc++-v3/testsuite/std/ranges/zip_transform/1.cc
index 20abdcb..9a0ad38 100644
--- a/libstdc++-v3/testsuite/std/ranges/zip_transform/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/zip_transform/1.cc
@@ -9,6 +9,23 @@
 namespace ranges = std::ranges;
 namespace views = std::views;
 
+template<typename T>
+concept can_zip_transform = requires (T t) {
+  views::zip_transform(std::forward<T>(t));
+};
+
+static_assert(!can_zip_transform<int>);
+
+struct NonMovable {
+  NonMovable(NonMovable&&) = delete;
+};
+
+static_assert(!can_zip_transform<NonMovable>);
+static_assert(!can_zip_transform<NonMovable&>);
+
+static_assert(!can_zip_transform<void(*)()>);
+static_assert(can_zip_transform<int(&(*)())[3]>);
+
 constexpr bool
 test01()
 {
@@ -46,6 +63,10 @@ test01()
   VERIFY( ranges::size(z3) == 3 );
   VERIFY( ranges::equal(z3, (int[]){3, 6, 9}) );
 
+  auto z4 = views::zip_transform([] () { return 1; });
+  VERIFY( ranges::size(z4) == 0 );
+  static_assert( std::same_as<ranges::range_value_t<decltype(z4)>, int> );
+
   return true;
 }