Fix compiler crash caused by implicit RHS quantification in type synonyms (#24470)
Replaced compiler crash with a proper error message:
type SynBad :: forall k. k -> Type
type SynBad = Proxy :: j -> Type
T24470a.hs:7:24: error: [GHC-84925]
• Type variable ‘j’ in the right-hand side of type synonym ‘SynBad’ is out of arity
• In the type synonym declaration for ‘SynBad’