This prevented the default behavior of BaseExpression._resolve_output_field from error'ing out when such functions accepted both expressions from mixed types (e.g. SubStr(CharField, IntegerField, IntegerField)).
Co-authored-by: Srinivas Reddy Thatiparthy <thatiparthysreenivas@gmail.com> Co-authored-by: Nick Pope <nick.pope@flightdataservices.com>