[GHC] #12810: -Wredundant-constraints doesn't factor in associated type families

[GHC]

#12810: -Wredundant-constraints doesn't factor in associated type families
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 Well type families are allowed to not reduce. For example
 {{{
 *M> :t undefined :: T Int
 undefined :: T Int :: T Int
 }}}
 there's nothing stopping you from writing `T Int`, regardless of whether
 there is an instance `C Int`. So GHC's warning is consistent with that.

 In your comment, the problem doesn't seem to be the type family, but
 rather the lack of any methods. An empty class body gives the same
 warning.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12810#comment:2>
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