def _simplified_return_type(self):
        """
        The NPM callconv has already converted simplified optional types.
        We can simply use the value type from it.
        """
        restype = self.fndesc.restype
        # Optional type
        if isinstance(restype, types.Optional):
            return restype.type
        else:
            return restype

        The NPM callconv has already converted simplified optional types.
        We can simply use the value type from it.
        
numba/core/callwrapper.py