[GHC] #3691: Error message doesn't mention necessary extension in warning

[GHC]

#3691: Error message doesn't mention necessary extension in warning
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  Reporter:  arsenm                             |          Owner:                  
      Type:  feature request                    |         Status:  reopened        
  Priority:  normal                             |      Milestone:                  
 Component:  Compiler                           |        Version:  6.10.4          
Resolution:                                     |       Keywords:                  
Difficulty:                                     |             Os:  Unknown/Multiple
  Testcase:                                     |   Architecture:  Unknown/Multiple
   Failure:  Incorrect warning at compile-time  |  
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Comment (by arsenm):

 What if there were only a suggestion (perhaps you meant to use
 ScopedTypeVariables??) if there is some type error concerning a type you
 use the same name for when it would have been out of scope. For instances,
 in the example, it complains that it can't deduce (Print a, ShowPred a
 flag1, Print' flag1 a), but it suggests adding (Print' flag a). In this
 situation at least, I think it would make sense to see that you tried to
 use the same name "flag" in 2 places, and the name "flag" was a problem in
 type checking then it might be a good suggestion. If everything is fine
 without it, then the warning might be unnecessary.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3691#comment:6>
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