[GHC] #11438: Code does not compile without ScopedTypeVariables

[GHC]

#11438: Code does not compile without ScopedTypeVariables
-------------------------------------+-------------------------------------
        Reporter:  wereHamster       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 This is not a bug, but we could do much better in suggesting
 `ScopedTypeVariables`. The reason that the change makes a difference is
 that you refer to the type variable `sublayout` from within a method
 definition in your instance. You need `ScopedTypeVariables` to bring the
 instance's type variables into scope.

 An easy way of suggesting `ScopedTypeVariables` just came to mind: pretend
 the extension is always on. When looking up a type variable, if the
 extension is off but the variable would be in scope otherwise, suggest the
 extension, while returning a lookup failure (because the variable really
 isn't in scope!). Getting caught on `ScopedTypeVariables` is a fairly
 common occurrence in my experience, so I think it's worth putting in a bit
 of effort to do better here. (Even better would be to look for type
 variables in a signature that's missing a `forall` to suggest adding the
 `forall`, but that can be a separate task.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11438#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler