[GHC] #7279: warning for unused type variables in instance contexts; -fwarn-unreachable-type-variables?

[GHC]

#7279: warning for unused type variables in instance contexts; -fwarn-unreachable-
type-variables?
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 Reporter:  nfrisby           |          Owner:                         
     Type:  feature request   |         Status:  new                    
 Priority:  normal            |      Component:  Compiler (Type checker)
  Version:  7.6.1             |       Keywords:                         
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple       
  Failure:  None/Unknown      |       Testcase:                         
Blockedby:                    |       Blocking:                         
  Related:                    |  
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 I just spend 90 minutes tracking down what ended up being a typo
 introduced via find-and-replace.

 I accidentally introduced a spurious constraint on an instance context:

 {{{
 instance (Monoid m, Context t) => Class t where …
 }}}

 I would like to be warned about constraints on variables that certainly
 have no connection to variables in the instance head. {{{-fwarn-
 unreachable-type-variables}}}?

 This might also help catch those signatures where all occurrences of type
 variable are as index arguments to a type family, rendering the functional
 unusable because of "ambiguous type variable" errors at every call site.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7279>
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