Adjusting type_caster<std::reference_wrapper<T>> to support const/non-const propagation in cast_op.

include/pybind11/cast.h

@@ -960,9 +960,14 @@ template <typename type> class type_caster<std::reference_wrapper<type>> {
 private:
     using caster_t = make_caster<type>;
     caster_t subcaster;
-    using subcaster_cast_op_type = typename caster_t::template cast_op_type<type>;
-    static_assert(std::is_same<typename std::remove_const<type>::type &, subcaster_cast_op_type>::value,
-            "std::reference_wrapper<T> caster requires T to have a caster with an `T &` operator");
+    using reference_t = type&;
+    using subcaster_cast_op_type =
+        typename caster_t::template cast_op_type<reference_t>;
+
+    static_assert(std::is_same<typename std::remove_const<type>::type &, subcaster_cast_op_type>::value ||
+                  std::is_same<reference_t, subcaster_cast_op_type>::value,
+                  "std::reference_wrapper<T> caster requires T to have a caster with an "
+                  "`operator T &()` or `operator const T &()`");
 public:
     bool load(handle src, bool convert) { return subcaster.load(src, convert); }
     static constexpr auto name = caster_t::name;
@@ -973,7 +978,7 @@ template <typename type> class type_caster<std::reference_wrapper<type>> {
         return caster_t::cast(&src.get(), policy, parent);
     }
     template <typename T> using cast_op_type = std::reference_wrapper<type>;
-    operator std::reference_wrapper<type>() { return subcaster.operator subcaster_cast_op_type&(); }
+    operator std::reference_wrapper<type>() { return cast_op<type &>(subcaster); }
 };
 
 #define PYBIND11_TYPE_CASTER(type, py_name) \

tests/test_builtin_casters.cpp

@@ -15,6 +15,49 @@
 #  pragma warning(disable: 4127) // warning C4127: Conditional expression is constant
 #endif
 
+struct ConstRefCasted {
+  int tag;
+};
+
+PYBIND11_NAMESPACE_BEGIN(pybind11)
+PYBIND11_NAMESPACE_BEGIN(detail)
+template <>
+class type_caster<ConstRefCasted> {
+ public:
+  static constexpr auto name = _<ConstRefCasted>();
+
+  // Input is unimportant, a new value will always be constructed based on the
+  // cast operator.
+  bool load(handle, bool) { return true; }
+
+  operator ConstRefCasted&&() { value = {1}; return std::move(value); }
+  operator ConstRefCasted&() { value = {2}; return value; }
+  operator ConstRefCasted*() { value = {3}; return &value; }
+
+  operator const ConstRefCasted&() { value = {4}; return value; }
+  operator const ConstRefCasted*() { value = {5}; return &value; }
+
+  // custom cast_op to explicitly propagate types to the conversion operators.
+  template <typename T_>
+  using cast_op_type =
+      /// const
+      conditional_t<
+          std::is_same<remove_reference_t<T_>, const ConstRefCasted*>::value, const ConstRefCasted*,
+      conditional_t<
+          std::is_same<T_, const ConstRefCasted&>::value, const ConstRefCasted&,
+      /// non-const
+      conditional_t<
+          std::is_same<remove_reference_t<T_>, ConstRefCasted*>::value, ConstRefCasted*,
+      conditional_t<
+          std::is_same<T_, ConstRefCasted&>::value, ConstRefCasted&,
+          /* else */ConstRefCasted&&>>>>;
+
+ private:
+  ConstRefCasted value = {0};
+};
+PYBIND11_NAMESPACE_END(detail)
+PYBIND11_NAMESPACE_END(pybind11)
+
 TEST_SUBMODULE(builtin_casters, m) {
     // test_simple_string
     m.def("string_roundtrip", [](const char *s) { return s; });
@@ -147,6 +190,17 @@ TEST_SUBMODULE(builtin_casters, m) {
     // test_reference_wrapper
     m.def("refwrap_builtin", [](std::reference_wrapper<int> p) { return 10 * p.get(); });
     m.def("refwrap_usertype", [](std::reference_wrapper<UserType> p) { return p.get().value(); });
+    m.def("refwrap_usertype_const", [](std::reference_wrapper<const UserType> p) { return p.get().value(); });
+
+    m.def("refwrap_lvalue", []() -> std::reference_wrapper<UserType> {
+        static UserType x(1);
+        return std::ref(x);
+    });
+    m.def("refwrap_lvalue_const", []() -> std::reference_wrapper<const UserType> {
+        static UserType x(1);
+        return std::cref(x);
+    });
+
     // Not currently supported (std::pair caster has return-by-value cast operator);
     // triggers static_assert failure.
     //m.def("refwrap_pair", [](std::reference_wrapper<std::pair<int, int>>) { });
@@ -189,4 +243,14 @@ TEST_SUBMODULE(builtin_casters, m) {
         py::object o = py::cast(v);
         return py::cast<void *>(o) == v;
     });
+
+    // Tests const/non-const propagation in cast_op.
+    m.def("takes", [](ConstRefCasted x) { return x.tag; });
+    m.def("takes_move", [](ConstRefCasted&& x) { return x.tag; });
+    m.def("takes_ptr", [](ConstRefCasted* x) { return x->tag; });
+    m.def("takes_ref", [](ConstRefCasted& x) { return x.tag; });
+    m.def("takes_ref_wrap", [](std::reference_wrapper<ConstRefCasted> x) { return x.get().tag; });
+    m.def("takes_const_ptr", [](const ConstRefCasted* x) { return x->tag; });
+    m.def("takes_const_ref", [](const ConstRefCasted& x) { return x.tag; });
+    m.def("takes_const_ref_wrap", [](std::reference_wrapper<const ConstRefCasted> x) { return x.get().tag; });
 }

tests/test_builtin_casters.py

@@ -315,6 +315,7 @@ def test_reference_wrapper():
     """std::reference_wrapper for builtin and user types"""
     assert m.refwrap_builtin(42) == 420
     assert m.refwrap_usertype(UserType(42)) == 42
+    assert m.refwrap_usertype_const(UserType(42)) == 42
 
     with pytest.raises(TypeError) as excinfo:
         m.refwrap_builtin(None)
@@ -324,6 +325,9 @@ def test_reference_wrapper():
         m.refwrap_usertype(None)
     assert "incompatible function arguments" in str(excinfo.value)
 
+    assert m.refwrap_lvalue().value == 1
+    assert m.refwrap_lvalue_const().value == 1
+
     a1 = m.refwrap_list(copy=True)
     a2 = m.refwrap_list(copy=True)
     assert [x.value for x in a1] == [2, 3]
@@ -421,3 +425,21 @@ def test_int_long():
 
 def test_void_caster_2():
     assert m.test_void_caster()
+
+
+def test_const_ref_caster():
+    """Verifies that const-ref is propagated through type_caster cast_op.
+    The returned ConstRefCasted type is a mimimal type that is constructed to
+    reference the casting mode used.
+    """
+    x = False
+    assert m.takes(x) == 1
+    assert m.takes_move(x) == 1
+
+    assert m.takes_ptr(x) == 3
+    assert m.takes_ref(x) == 2
+    assert m.takes_ref_wrap(x) == 2
+
+    assert m.takes_const_ptr(x) == 5
+    assert m.takes_const_ref(x) == 4
+    assert m.takes_const_ref_wrap(x) == 4