Downsides to ScopedTypeVariables

Question

What are the downsides to ScopedTypeVariables, if there are any? Why isn't it on by default? Does it lead to worse inference? Are there edge cases where it fails? Is it significantly harder to implement in GHC?

Answer 1

It's also because it changes the semantics of a program (combined with other extensions). Consider

{-# LANGUAGE RankNTypes #-}
foo :: forall a . (a -> a) -> (a -> a)
foo = bar
  where
    bar :: a -> a
    bar = id

It compiles fine, but with ScopedTypeVariables it even fails to compile.