Escaping skolems (e.g. with runST) are not mentioned in GHC >= 9.2

[Innf107]

In GHC 9.0 and below, the following code would produce an error message that mentions the phrase type variable 's' would escape its scope.

runST (newSTRef 5)

<interactive>:3:8: error:
    • Couldn't match type ‘a’ with ‘STRef s Integer’
      Expected: ST s a
        Actual: ST s (STRef s Integer)
        because type variable ‘s’ would escape its scope
      This (rigid, skolem) type variable is bound by
        a type expected by the context:
          forall s. ST s a
        at <interactive>:3:7-18
    • In the first argument of ‘runST’, namely ‘(newSTRef 5)’
      In the expression: runST (newSTRef 5)
      In an equation for ‘it’: it = runST (newSTRef 5)
    • Relevant bindings include it :: a (bound at <interactive>:3:1)

GHC >= 9.2 only reports the type mismatch and doesn't mention anything about escaping scopes anymore.

<interactive>:3:8: error: [GHC-25897]
    • Couldn't match type ‘a’ with ‘STRef s Integer’
      Expected: ST s a
        Actual: ST s (STRef s Integer)
      ‘a’ is a rigid type variable bound by
        the inferred type of it :: a
        at <interactive>:3:1-18
    • In the first argument of ‘runST’, namely ‘(newSTRef 5)’
      In the expression: runST (newSTRef 5)
      In an equation for ‘it’: it = runST (newSTRef 5)
    • Relevant bindings include it :: a (bound at <interactive>:3:1)

That s would escape its scope is arguably the most important part of the error message, so if this is reverted, that line should probably be moved before the type mismatch. Especially in cases like this with runST, the type mismatch is not very meaningful without that piece of information.

For example:

<interactive>:3:8: error: [GHC-25897]
    • The rigid type variable 's' would escape its scope
    • Couldn't match type ‘a’ with ‘STRef s Integer’
      Expected: ST s a
        Actual: ST s (STRef s Integer)
      ‘a’ is a rigid type variable bound by
        the inferred type of it :: a
        at <interactive>:3:1-18
    • In the first argument of ‘runST’, namely ‘(newSTRef 5)’
      In the expression: runST (newSTRef 5)
      In an equation for ‘it’: it = runST (newSTRef 5)
    • Relevant bindings include it :: a (bound at <interactive>:3:1)