Function confirm_callable_candidate

fn confirm_callable_candidate<'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>,
    obligation: &ProjectionTermObligation<'tcx>,
    fn_sig: PolyFnSig<'tcx>,
    flag: TupleArgumentsFlag,
    fn_host_effect: Const<'tcx>,
) -> Progress<'tcx>