feat: change array_caster in pybind11/stl.h to support value types that are not default-constructible.

include/pybind11/stl.h

@@ -11,11 +11,14 @@
 
 #include "pybind11.h"
 #include "detail/common.h"
+#include "detail/descr.h"
+#include "detail/type_caster_base.h"
 
 #include <deque>
 #include <initializer_list>
 #include <list>
 #include <map>
+#include <memory>
 #include <ostream>
 #include <set>
 #include <unordered_map>
@@ -349,36 +352,65 @@ struct type_caster<std::deque<Type, Alloc>> : list_caster<std::deque<Type, Alloc
 template <typename Type, typename Alloc>
 struct type_caster<std::list<Type, Alloc>> : list_caster<std::list<Type, Alloc>, Type> {};
 
+template <typename ArrayType, typename V, size_t... I>
+ArrayType vector_to_array_impl(V &&v, index_sequence<I...>) {
+    return {{std::move(v[I])...}};
+}
+
+// Based on https://en.cppreference.com/w/cpp/container/array/to_array
+template <typename ArrayType, size_t N, typename V>
+ArrayType vector_to_array(V &&v) {
+    return vector_to_array_impl<ArrayType, V>(std::forward<V>(v), make_index_sequence<N>{});
+}
+
 template <typename ArrayType, typename Value, bool Resizable, size_t Size = 0>
 struct array_caster {
     using value_conv = make_caster<Value>;
 
 private:
-    template <bool R = Resizable>
-    bool require_size(enable_if_t<R, size_t> size) {
-        if (value.size() != size) {
-            value.resize(size);
+    std::unique_ptr<ArrayType> value;
+
+    template <bool R = Resizable, enable_if_t<R, int> = 0>
+    bool convert_elements(handle seq, bool convert) {
+        auto l = reinterpret_borrow<sequence>(seq);
+        value.reset(new ArrayType{});
+        // Using `resize` to preserve the behavior exactly as it was before PR #5305
+        // For the `resize` to work, `Value` must be default constructible.
+        // For `std::valarray`, this is a requirement:
+        // https://en.cppreference.com/w/cpp/named_req/NumericType
+        value->resize(l.size());
+        size_t ctr = 0;
+        for (const auto &it : l) {
+            value_conv conv;
+            if (!conv.load(it, convert)) {
+                return false;
+            }
+            (*value)[ctr++] = cast_op<Value &&>(std::move(conv));
         }
         return true;
     }
-    template <bool R = Resizable>
-    bool require_size(enable_if_t<!R, size_t> size) {
-        return size == Size;
-    }
 
+    template <bool R = Resizable, enable_if_t<!R, int> = 0>
     bool convert_elements(handle seq, bool convert) {
         auto l = reinterpret_borrow<sequence>(seq);
-        if (!require_size(l.size())) {
+        if (l.size() != Size) {
             return false;
         }
-        size_t ctr = 0;
-        for (const auto &it : l) {
+        // The `temp` storage is needed to support `Value` types that are not
+        // default-constructible.
+        // Deliberate choice: no template specializations, for simplicity, and
+        // because the compile time overhead for the specializations is deemed
+        // more significant than the runtime overhead for the `temp` storage.
+        std::vector<Value> temp;
+        temp.reserve(l.size());
+        for (auto it : l) {
             value_conv conv;
             if (!conv.load(it, convert)) {
                 return false;
             }
-            value[ctr++] = cast_op<Value &&>(std::move(conv));
+            temp.emplace_back(cast_op<Value &&>(std::move(conv)));
         }
+        value.reset(new ArrayType(vector_to_array<ArrayType, Size>(std::move(temp))));
         return true;
     }
 
@@ -416,12 +448,36 @@ struct array_caster {
         return l.release();
     }
 
-    PYBIND11_TYPE_CASTER(ArrayType,
-                         const_name<Resizable>(const_name(""), const_name("Annotated["))
-                             + const_name("list[") + value_conv::name + const_name("]")
-                             + const_name<Resizable>(const_name(""),
-                                                     const_name(", FixedSize(")
-                                                         + const_name<Size>() + const_name(")]")));
+    // Code copied from PYBIND11_TYPE_CASTER macro.
+    // Intentionally preserving the behavior exactly as it was before PR #5305
+    template <typename T_, enable_if_t<std::is_same<ArrayType, remove_cv_t<T_>>::value, int> = 0>
+    static handle cast(T_ *src, return_value_policy policy, handle parent) {
+        if (!src) {
+            return none().release();
+        }
+        if (policy == return_value_policy::take_ownership) {
+            auto h = cast(std::move(*src), policy, parent);
+            delete src; // WARNING: Assumes `src` was allocated with `new`.
+            return h;
+        }
+        return cast(*src, policy, parent);
+    }
+
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    operator ArrayType *() { return &(*value); }
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    operator ArrayType &() { return *value; }
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    operator ArrayType &&() && { return std::move(*value); }
+
+    template <typename T_>
+    using cast_op_type = movable_cast_op_type<T_>;
+
+    static constexpr auto name
+        = const_name<Resizable>(const_name(""), const_name("Annotated[")) + const_name("list[")
+          + value_conv::name + const_name("]")
+          + const_name<Resizable>(
+              const_name(""), const_name(", FixedSize(") + const_name<Size>() + const_name(")]"));
 };
 
 template <typename Type, size_t Size>

tests/test_stl.cpp

@@ -201,6 +201,23 @@ TEST_SUBMODULE(stl, m) {
     m.def("cast_array", []() { return std::array<int, 2>{{1, 2}}; });
     m.def("load_array", [](const std::array<int, 2> &a) { return a[0] == 1 && a[1] == 2; });
 
+    struct NoDefaultCtor {
+        explicit constexpr NoDefaultCtor(int val) : val{val} {}
+        int val;
+    };
+
+    struct NoDefaultCtorArray {
+        explicit constexpr NoDefaultCtorArray(int i)
+            : arr{{NoDefaultCtor(10 + i), NoDefaultCtor(20 + i)}} {}
+        std::array<NoDefaultCtor, 2> arr;
+    };
+
+    // test_array_no_default_ctor
+    py::class_<NoDefaultCtor>(m, "NoDefaultCtor").def_readonly("val", &NoDefaultCtor::val);
+    py::class_<NoDefaultCtorArray>(m, "NoDefaultCtorArray")
+        .def(py::init<int>())
+        .def_readwrite("arr", &NoDefaultCtorArray::arr);
+
     // test_valarray
     m.def("cast_valarray", []() { return std::valarray<int>{1, 4, 9}; });
     m.def("load_valarray", [](const std::valarray<int> &v) {

tests/test_stl.py

@@ -48,6 +48,13 @@ def test_array(doc):
     )
 
 
+def test_array_no_default_ctor():
+    lst = m.NoDefaultCtorArray(3)
+    assert [e.val for e in lst.arr] == [13, 23]
+    lst.arr = m.NoDefaultCtorArray(4).arr
+    assert [e.val for e in lst.arr] == [14, 24]
+
+
 def test_valarray(doc):
     """std::valarray <-> list"""
     lst = m.cast_valarray()