[PATCH v2] libstdc++: Fix views::zip_transform constraints for empty range pack [PR111138]

[Tomasz Kamiński]

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
---
 Move the empty-ranges case into separate `operator()`.
 Add missing `is_object<invoke_result_t<....>>`.

 Tested on x86_64-linux. OK for trunk?

 libstdc++-v3/include/std/ranges               | 16 +++++++++-----
 .../testsuite/std/ranges/zip_transform/1.cc   | 21 +++++++++++++++++++
 2 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index e21f5284b46..97d22a12dc6 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -5331,15 +5331,21 @@ namespace views::__adaptor
 
     struct _ZipTransform
     {
+      template<typename _Fp>
+       requires move_constructible<decay_t<_Fp>> && 
regular_invocable<decay_t<_Fp>&>
+         && is_object_v<decay_t<invoke_result_t<decay_t<_Fp>&>>>
+       constexpr auto
+       operator() [[nodiscard]] (_Fp&& __f) const
+       {
+         return views::empty<decay_t<invoke_result_t<decay_t<_Fp>&>>>;
+       }
+
       template<typename _Fp, typename... _Ts>
-       requires (sizeof...(_Ts) == 0) || 
__detail::__can_zip_transform_view<_Fp, _Ts...>
+       requires (sizeof...(_Ts) != 0) && 
__detail::__can_zip_transform_view<_Fp, _Ts...>
        constexpr auto
        operator() [[nodiscard]] (_Fp&& __f, _Ts&&... __ts) const
        {
-         if constexpr (sizeof...(_Ts) == 0)
-           return views::empty<decay_t<invoke_result_t<decay_t<_Fp>&>>>;
-         else
-           return zip_transform_view(std::forward<_Fp>(__f), 
std::forward<_Ts>(__ts)...);
+          return zip_transform_view(std::forward<_Fp>(__f), 
std::forward<_Ts>(__ts)...);
        }
     };
 
diff --git a/libstdc++-v3/testsuite/std/ranges/zip_transform/1.cc 
b/libstdc++-v3/testsuite/std/ranges/zip_transform/1.cc
index 20abdcba0f8..9a0ad3814e6 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;
 }
 
-- 
2.48.1