Add lvalue ref-qualified cpp_function constructors (#2213)

* added overload for l-value ref-qualified methods

* Added test.
Before, the code would have failed to build.

include/pybind11/pybind11.h

@@ -72,20 +72,38 @@ class cpp_function : public function {
                    (detail::function_signature_t<Func> *) nullptr, extra...);
     }
 
-    /// Construct a cpp_function from a class method (non-const)
+    /// Construct a cpp_function from a class method (non-const, no ref-qualifier)
     template <typename Return, typename Class, typename... Arg, typename... Extra>
     cpp_function(Return (Class::*f)(Arg...), const Extra&... extra) {
         initialize([f](Class *c, Arg... args) -> Return { return (c->*f)(args...); },
                    (Return (*) (Class *, Arg...)) nullptr, extra...);
     }
 
-    /// Construct a cpp_function from a class method (const)
+    /// Construct a cpp_function from a class method (non-const, lvalue ref-qualifier)
+    /// A copy of the overload for non-const functions without explicit ref-qualifier
+    /// but with an added `&`.
+    template <typename Return, typename Class, typename... Arg, typename... Extra>
+    cpp_function(Return (Class::*f)(Arg...)&, const Extra&... extra) {
+        initialize([f](Class *c, Arg... args) -> Return { return (c->*f)(args...); },
+                   (Return (*) (Class *, Arg...)) nullptr, extra...);
+    }
+
+    /// Construct a cpp_function from a class method (const, no ref-qualifier)
     template <typename Return, typename Class, typename... Arg, typename... Extra>
     cpp_function(Return (Class::*f)(Arg...) const, const Extra&... extra) {
         initialize([f](const Class *c, Arg... args) -> Return { return (c->*f)(args...); },
                    (Return (*)(const Class *, Arg ...)) nullptr, extra...);
     }
 
+    /// Construct a cpp_function from a class method (const, lvalue ref-qualifier)
+    /// A copy of the overload for const functions without explicit ref-qualifier
+    /// but with an added `&`.
+    template <typename Return, typename Class, typename... Arg, typename... Extra>
+    cpp_function(Return (Class::*f)(Arg...) const&, const Extra&... extra) {
+        initialize([f](const Class *c, Arg... args) -> Return { return (c->*f)(args...); },
+                   (Return (*)(const Class *, Arg ...)) nullptr, extra...);
+    }
+
     /// Return the function name
     object name() const { return attr("__name__"); }
 

tests/test_methods_and_attributes.cpp

@@ -207,6 +207,14 @@ class RegisteredDerived : public UnregisteredBase {
     double sum() const { return rw_value + ro_value; }
 };
 
+// Test explicit lvalue ref-qualification
+struct RefQualified {
+    int value = 0;
+
+    void refQualified(int other) & { value += other; }
+    int constRefQualified(int other) const & { return value + other; }
+};
+
 TEST_SUBMODULE(methods_and_attributes, m) {
     // test_methods_and_attributes
     py::class_<ExampleMandA> emna(m, "ExampleMandA");
@@ -457,4 +465,11 @@ TEST_SUBMODULE(methods_and_attributes, m) {
     m.def("custom_caster_destroy_const", []() -> const DestructionTester * { return new DestructionTester(); },
             py::return_value_policy::take_ownership); // Likewise (const doesn't inhibit destruction)
     m.def("destruction_tester_cstats", &ConstructorStats::get<DestructionTester>, py::return_value_policy::reference);
+
+    // test_methods_and_attributes
+    py::class_<RefQualified>(m, "RefQualified")
+        .def(py::init<>())
+        .def_readonly("value", &RefQualified::value)
+        .def("refQualified", &RefQualified::refQualified)
+        .def("constRefQualified", &RefQualified::constRefQualified);
 }

tests/test_methods_and_attributes.py

@@ -510,3 +510,14 @@ def test_custom_caster_destruction():
 
     # Make sure we still only have the original object (from ..._no_destroy()) alive:
     assert cstats.alive() == 1
+
+
+def test_ref_qualified():
+    """Tests that explicit lvalue ref-qualified methods can be called just like their
+    non ref-qualified counterparts."""
+
+    r = m.RefQualified()
+    assert r.value == 0
+    r.refQualified(17)
+    assert r.value == 17
+    assert r.constRefQualified(23) == 40