Fail static_assert when trying to reference non-referencable types

The previous commit to address #392 triggers a compiler warning about
returning a reference to a local variable, which is *not* a false alarm:
the following:

    py::cast<int &>(o)

(which happens internally in an overload declaration) really is
returning a reference to a local, because the cast operators for the
type_caster for numeric types returns a reference to its own member.

This commit adds a static_assert to make that a compilation failure
rather than returning a reference into about-to-be-freed memory.

Incidentally, this is also a fix for #219, which is exactly the same
issue: we can't reference numeric primitives that are cast from
wrappers around python numeric types.

include/pybind11/cast.h

@@ -855,10 +855,22 @@ template <typename T> struct move_if_unreferenced<T, typename std::enable_if<
     >::type> : std::true_type {};
 template <typename T> using move_never = std::integral_constant<bool, !move_always<T>::value && !move_if_unreferenced<T>::value>;
 
+// Detect whether returning a `type` from a cast on type's type_caster is going to result in a
+// reference or pointer to a local variable of the type_caster.  Basically, only
+// non-reference/pointer `type`s and reference/pointers from a type_caster_generic are safe;
+// everything else returns a reference/pointer to a local variable.
+template <typename type> using cast_is_temporary_value_reference = bool_constant<
+    (std::is_reference<type>::value || std::is_pointer<type>::value) &&
+    !std::is_base_of<type_caster_generic, make_caster<type>>::value
+>;
+
+
 NAMESPACE_END(detail)
 
 template <typename T> T cast(const handle &handle) {
     using type_caster = detail::make_caster<T>;
+    static_assert(!detail::cast_is_temporary_value_reference<T>::value,
+            "Unable to cast type to reference: value is local to type caster");
     type_caster conv;
     if (!conv.load(handle, true)) {
 #if defined(NDEBUG)

tests/test_issues.cpp

@@ -208,7 +208,9 @@ void init_issues(py::module &m) {
     public:
         using OverrideTest::OverrideTest;
         int int_value() override { PYBIND11_OVERLOAD(int, OverrideTest, int_value); }
-        int &int_ref() override { PYBIND11_OVERLOAD(int &, OverrideTest, int_ref); }
+        // Not allowed (uncommenting should hit a static_assert failure): we can't get a reference
+        // to a python numeric value, since we only copy values in the numeric type caster:
+//      int &int_ref() override { PYBIND11_OVERLOAD(int &, OverrideTest, int_ref); }
         A A_value() override { PYBIND11_OVERLOAD(A, OverrideTest, A_value); }
         A &A_ref() override { PYBIND11_OVERLOAD(A &, OverrideTest, A_ref); }
     };
@@ -217,9 +219,10 @@ void init_issues(py::module &m) {
     py::class_<OverrideTest, PyOverrideTest>(m2, "OverrideTest")
         .def(py::init<int>())
         .def("int_value", &OverrideTest::int_value)
-        .def("int_ref", &OverrideTest::int_ref)
+//      .def("int_ref", &OverrideTest::int_ref)
         .def("A_value", &OverrideTest::A_value)
         .def("A_ref", &OverrideTest::A_ref);
+
 }
 
 // MSVC workaround: trying to use a lambda here crashes MSCV

tests/test_issues.py

@@ -171,9 +171,10 @@ def test_override_ref():
     from pybind11_tests.issues import OverrideTest
     o = OverrideTest(42)
 
-    i = o.int_ref()
+    # Not allowed (see associated .cpp comment)
+    #i = o.int_ref()
+    #assert o.int_ref() == 42
     assert o.int_value() == 42
-    assert o.int_ref() == 42
 
     assert o.A_value().value == 99
     a = o.A_ref()