Ensure we are not leaking a Python error is something is wrong elsewhere (e.g. UB, or bug in Python beta testing).

See also: #5879 (comment)

Author: rwgk

include/pybind11/cast.h

@@ -285,8 +285,10 @@ struct type_caster<T, enable_if_t<std::is_arithmetic<T>::value && !is_std_char_t
             }
         }
 
-        // Python API reported an error
-        bool py_err = py_value == (py_type) -1 && PyErr_Occurred();
+        bool py_err = (PyErr_Occurred() != nullptr);
+        if (py_err) {
+            assert(py_value == static_cast<py_type>(-1));
+        }
 
         // Check to see if the conversion is valid (integers should match exactly)
         // Signed/unsigned checks happen elsewhere