Always call PyNumber_Index when casting from Python to a C++ integral type, also pre-3.8

Author: YannickJadoul

include/pybind11/cast.h

@@ -1029,12 +1029,30 @@ struct type_caster<T, enable_if_t<std::is_arithmetic<T>::value && !is_std_char_t
             return false;
         } else if (!convert && !PyIndex_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:
-            py_value = sizeof(T) <= sizeof(long)
-                ? (py_type) PyLong_AsLong(src.ptr())
-                : (py_type) PYBIND11_LONG_AS_LONGLONG(src.ptr());
+        } else {
+            handle obj = src;
+#if PY_VERSION_HEX < 0x03080000
+            bool do_decref = false;
+            if (PyIndex_Check(src.ptr())) {
+                PyObject *tmp = PyNumber_Index(src.ptr());
+                if (!tmp) {
+                    py_value = (py_type) -1;
+                }
+                do_decref = true;
+                obj = tmp;
+            }
+#endif
+            if (std::is_unsigned<py_type>::value) {
+                py_value = as_unsigned<py_type>(obj.ptr());
+            } else { // signed integer:
+                py_value = sizeof(T) <= sizeof(long)
+                    ? (py_type) PyLong_AsLong(obj.ptr())
+                    : (py_type) PYBIND11_LONG_AS_LONGLONG(obj.ptr());
+            }
+#if PY_VERSION_HEX < 0x03080000
+            if (do_decref)
+                obj.dec_ref();
+#endif
         }
 
         // Python API reported an error

tests/test_builtin_casters.py

@@ -289,11 +289,12 @@ def cant_convert(v):
     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
+
+    # Before Python 3.8, `int(obj)` does not pick up on `obj.__index__`, but pybind11
+    # "backports" this behavior.
+    assert convert(IndexedThought()) == 42
+    assert noconvert(IndexedThought()) == 42
+    cant_convert(RaisingThought())  # no fall-back to `__int__`if `__index__` raises
 
 
 def test_numpy_int_convert():