def resolve_input_arg_const(input_arg_idx):
        """
        Resolves an input arg to a constant (if possible)
        """
        input_arg_ty = called_args[input_arg_idx]

        # comparing to None?
        if isinstance(input_arg_ty, types.NoneType):
            return input_arg_ty

        # is it a kwarg default
        if isinstance(input_arg_ty, types.Omitted):
            val = input_arg_ty.value
            if isinstance(val, types.NoneType):
                return val
            elif val is None:
                return types.NoneType('none')

        # literal type, return the type itself so comparisons like `x == None`
        # still work as e.g. x = types.int64 will never be None/NoneType so
        # the branch can still be pruned
        return getattr(input_arg_ty, 'literal_type', Unknown())

        Resolves an input arg to a constant (if possible)
        
numba/core/analysis.py