[GHC] #10581: Recommend ScopedTypeVariables

[GHC]

#10581: Recommend ScopedTypeVariables
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              Reporter:  goldfire    |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.10.1
             Component:  Compiler    |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 The !ScopedTypeVariables extension is one of the few that doesn't
 recommend itself when it should be used.

 I recognize that this may be hard to do, but here is a strawman proposal:
 Always behave as if !ScopedTypeVariables is on at binding sites for type
 variables (in an annotation with `forall`). Then, at ''occurrences'' of
 type variables, check if the extension is on. If a type variable is in
 scope but the extension is off, remember that the user probably wants the
 extension, but then rename the type variable occurrence away from the in-
 scope one. If a type error ensues, we've remembered that the lack of
 !ScopedTypeVariables may be to blame, and we recommend turning it on.

 There may be better approaches, too.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10581>
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