Only allow integer type_caster to call __int__ method when conversion is allowed; always call __index__

include/pybind11/cast.h

@@ -1020,13 +1020,23 @@ struct type_caster<T, enable_if_t<std::is_arithmetic<T>::value && !is_std_char_t
         if (!src)
             return false;
 
+#if !defined(PYPY_VERSION)
+        auto index_check = [](PyObject *o) { return PyIndex_Check(o); };
+#else
+        // In PyPy 7.3.3, `PyIndex_Check` is implemented by calling `__index__`,
+        // while CPython only considers the existence of `nb_index`/`__index__`.
+        auto index_check = [](PyObject *o) { return hasattr(o, "__index__"); };
+#endif
+
         if (std::is_floating_point<T>::value) {
             if (convert || PyFloat_Check(src.ptr()))
                 py_value = (py_type) PyFloat_AsDouble(src.ptr());
             else
                 return false;
         } else if (PyFloat_Check(src.ptr())) {
             return false;
+        } else if (!convert && !index_check(src.ptr()) && !PYBIND11_LONG_CHECK(src.ptr())) {
+            return false;
         } else if (std::is_unsigned<py_type>::value) {
             py_value = as_unsigned<py_type>(src.ptr());
         } else { // signed integer:

tests/test_builtin_casters.cpp

@@ -98,6 +98,10 @@ TEST_SUBMODULE(builtin_casters, m) {
     m.def("i64_str", [](std::int64_t v) { return std::to_string(v); });
     m.def("u64_str", [](std::uint64_t v) { return std::to_string(v); });
 
+    // test_int_convert
+    m.def("int_passthrough", [](int arg) { return arg; });
+    m.def("int_passthrough_noconvert", [](int arg) { return arg; }, py::arg{}.noconvert());
+
     // test_tuple
     m.def("pair_passthrough", [](std::pair<bool, std::string> input) {
         return std::make_pair(input.second, input.first);

tests/test_builtin_casters.py

@@ -251,6 +251,68 @@ def test_integer_casting():
         assert "incompatible function arguments" in str(excinfo.value)
 
 
+def test_int_convert():
+    class DeepThought(object):
+        def __int__(self):
+            return 42
+
+    class ShallowThought(object):
+        pass
+
+    class FuzzyThought(object):
+        def __float__(self):
+            return 41.99999
+
+    class IndexedThought(object):
+        def __index__(self):
+            return 42
+
+    class RaisingThought(object):
+        def __index__(self):
+            raise ValueError
+
+        def __int__(self):
+            return 42
+
+    convert, noconvert = m.int_passthrough, m.int_passthrough_noconvert
+
+    def require_implicit(v):
+        pytest.raises(TypeError, noconvert, v)
+
+    def cant_convert(v):
+        pytest.raises(TypeError, convert, v)
+
+    assert convert(7) == 7
+    assert noconvert(7) == 7
+    cant_convert(3.14159)
+    assert convert(DeepThought()) == 42
+    require_implicit(DeepThought())
+    cant_convert(ShallowThought())
+    cant_convert(FuzzyThought())
+    if env.PY >= (3, 8):
+        # Before Python 3.8, `int(obj)` does not pick up on `obj.__index__`
+        assert convert(IndexedThought()) == 42
+        assert noconvert(IndexedThought()) == 42
+        cant_convert(RaisingThought())  # no fall-back to `__int__`if `__index__` raises
+
+
+def test_numpy_int_convert():
+    np = pytest.importorskip("numpy")
+
+    convert, noconvert = m.int_passthrough, m.int_passthrough_noconvert
+
+    def require_implicit(v):
+        pytest.raises(TypeError, noconvert, v)
+
+    # `np.intc` is an alias that corresponds to a C++ `int`
+    assert convert(np.intc(42)) == 42
+    assert noconvert(np.intc(42)) == 42
+
+    # The implicit conversion from np.float32 is undesirable but currently accepted.
+    assert convert(np.float32(3.14159)) == 3
+    require_implicit(np.float32(3.14159))
+
+
 def test_tuple(doc):
     """std::pair <-> tuple & std::tuple <-> tuple"""
     assert m.pair_passthrough((True, "test")) == ("test", True)