Improve optimisation of pattern synonym matching
Currently GHC does a very poor job of optimising pattern matches which include pattern synonyms.
It would be good to modify
compiler/deSugar/Match.hs to be a lot smarter about when it is safe to group two pattern matches together.
Grouping was originally disabled as it is a bit trickier than originally thought. See #11224 (closed) for details.
The rule was originally to group two pattern matches together
PgSyn p1 and
PgSyn p2 when
p1 == p2. This rule wasn't safe when
p1 had a polymorphic return type and thus could have a different type on each branch even though syntactically identical. A good solution to this ticket would come up with a smarter rule about when it is and isn't safe to group two together in the same spirit as the more complicated rules for view patterns.