Bottom line on this is even with IRST a plane needs to count on that IRST outranging the very powerful AESA radars on F-35 and F-22.
If an F-35 sees a Typhoon with IRST first it will use that advantage to get into a favorable kill position that is outside the field of view of both the Typhoon's radar and IRST. It can launch a BVRAAM and passively guide it to target. The F-35 isn't in FOV of radar or IRST so there isn't any seeing it when it launches, and it isn't detecting the passive missile to have an early track back to the launching platform. All it gets is the RWR going off when that missile goes active, and then seconds to evade and survive.
The notion that a plane having a RWR go off because an AMRAAM just went active and is seconds away would be met with cool calculations to determine the exact location of the launching platform is ridiculous, all that pilot doing is trying to live to the next minute while losing a lot of energy in the process making him even more vulnerable for the next shot.
I'm not doubting the value of IRST systems, but the belief they are some panacea to easily defeat 5th gen LO aircraft is quite misplaced, they are only an advantage if they see the F-22/F-35 first.