[gcc r15-8057] libstdc++: Fix views::zip_transform constraints for empty range pack [PR111138]

[Tomasz Kaminski]

https://gcc.gnu.org/g:5abe571e0276fafcc6eed27c27abb28943e67c6f

commit r15-8057-g5abe571e0276fafcc6eed27c27abb28943e67c6f
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Fri Mar 7 11:54:38 2025 +0100

    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>

Diff:
---
 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 ef277b81bd3c..34c6f113e216 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -5333,15 +5333,21 @@ 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 20abdcba0f85..9a0ad3814e66 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;
 }