fix: regression in #3271

Author: henryiii

include/pybind11/pybind11.h

@@ -1967,26 +1967,26 @@ struct iterator_state {
 
 // Note: these helpers take the iterator by non-const reference because some
 // iterators in the wild can't be dereferenced when const.
-template <typename Iterator>
+template <typename Iterator, typename ResultType = decltype(*std::declval<Iterator>())>
 struct iterator_access {
-    using result_type = decltype((*std::declval<Iterator>()));
+    using result_type = ResultType;
     // NOLINTNEXTLINE(readability-const-return-type) // PR #3263
     result_type operator()(Iterator &it) const {
         return *it;
     }
 };
 
-template <typename Iterator>
+template <typename Iterator, typename ResultType = decltype((*std::declval<Iterator>()).first) >
 struct iterator_key_access {
-    using result_type = decltype(((*std::declval<Iterator>()).first));
+    using result_type = ResultType;
     result_type operator()(Iterator &it) const {
         return (*it).first;
     }
 };
 
-template <typename Iterator>
+template <typename Iterator, typename ResultType = decltype((*std::declval<Iterator>()).second)>
 struct iterator_value_access {
-    using result_type = decltype(((*std::declval<Iterator>()).second));
+    using result_type = ResultType;
     result_type operator()(Iterator &it) const {
         return (*it).second;
     }

tests/test_sequences_and_iterators.cpp

@@ -306,6 +306,7 @@ TEST_SUBMODULE(sequences_and_iterators, m) {
     public:
         explicit IntPairs(std::vector<std::pair<int, int>> data) : data_(std::move(data)) {}
         const std::pair<int, int>* begin() const { return data_.data(); }
+        const std::pair<int, int>* end() const { return data_.data() + data_.size(); }
     private:
         std::vector<std::pair<int, int>> data_;
     };
@@ -320,6 +321,10 @@ TEST_SUBMODULE(sequences_and_iterators, m) {
         .def("nonzero_values", [](const IntPairs& s) {
             return py::make_value_iterator(NonZeroIterator<std::pair<int, int>>(s.begin()), NonZeroSentinel());
         }, py::keep_alive<0, 1>())
+        // test iterator with keep_alive (doesn't work, but tests compile)
+        .def("make_iterator_keep_alive", [](IntPairs& self) {
+            return py::make_iterator(self, py::keep_alive<0, 1>());
+        }, py::keep_alive<0, 1>())
         ;
 
     // test_iterater_referencing

tests/test_sequences_and_iterators.py

@@ -21,6 +21,10 @@ def test_generalized_iterators():
     assert list(m.IntPairs([(1, 2), (2, 0), (0, 3), (4, 5)]).nonzero()) == [(1, 2)]
     assert list(m.IntPairs([(0, 3), (1, 2), (3, 4)]).nonzero()) == []
 
+    # keep_alive can't be attached
+    with pytest.raises(RuntimeError):
+        m.IntPairs([(1, 2), (3, 4), (0, 5)]).make_iterator_keep_alive()
+
     assert list(m.IntPairs([(1, 2), (3, 4), (0, 5)]).nonzero_keys()) == [1, 3]
     assert list(m.IntPairs([(1, 2), (2, 0), (0, 3), (4, 5)]).nonzero_keys()) == [1]
     assert list(m.IntPairs([(0, 3), (1, 2), (3, 4)]).nonzero_keys()) == []