Ticket #2523 (closed bug: fixed)
Improve warnings for use of "forall" without -XExistentialQuantification
Here's a GHCi session:
Prelude> let foo :: forall a. a; foo = undefined <interactive>:1:19: Illegal operator `.' in type `forall a . a' (Use -XTypeOperators to allow operators in types) Prelude> let foo :: forall a. Show a => a; foo = undefined <interactive>:1:28: parse error on input `=>'
Both of these errors could be improved, I think, by detecting the use of 'forall' as a type variable name, and suggesting -XExistentialQuantification if it's being used.