Suppose the following Python function:

def test_optional_result(x: int) -> Optional[int]:
    return x if x >= 0 else None

I would have expected the C# method to return a long?, but instead it's typed to return just long:

long TestOptionalResult(long x);

This means that it's not possible to test for null statically, like so:

switch (testModule.TestOptionalResult(-1))
{
    case null:
        // ...
        break;
    case var result:
        // ...
        break;
}

since the compiler (rightfully) emits CS0037, “Cannot convert null to 'long' because it is a non-nullable value type”. If one works around that by casting to object before testing:

//      V......V
switch ((object)testModule.TestOptionalResult(-1))
{
    case null:
        // ...
        break;
    case var result:
        // ...
        break;
}

the code compiles but results in the runtime exception:

CSnakes.Runtime.PythonInvocationException
Error converting Python object to int, check that the object was a Python int or that the value wasn't too large. See InnerException for details.
   at CSnakes.Runtime.CPython.CPythonAPI.PyLong_AsLongLong(PyObject p) in A:\CSnakes\main\src\CSnakes.Runtime\CPython\Long.cs:line 26

The inner exception reads:

CSnakes.Runtime.PythonRuntimeException
'NoneType' object cannot be interpreted as an integer
  Exception doesn't have a stacktrace

The same problem exists with input arguments that are typed as Optional of some type (when there's no default of None given).

It looks like the problem is with TypeReflection.AsPredefinedType, which simply looks at the type argument:

The existing nullability support is predicated on the presence of None as the default argument in ArgumentReflection.ArgumentSyntax:

The fix would be to consider Optional in the type hint as opposed to requiring a default of None. This is especially problematic for the return value.