[GHC] #11450: Associated types at wrong type in instance

[GHC]

#11450: Associated types at wrong type in instance
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       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:  #11449, #11451    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * related:   => #11449, #11451


Comment:

 Oh, I wasn't aware of #11449 and #11451. Those are pretty important
 context for navigating this.

 I don't have any objection (in principle) to requiring that the class
 instance types be the same as the associate types. All I ask is please
 allow me to write something like this if I want to:

 {{{#!hs
 instance C (Either a _) where
   type T (Either a _) = a
 }}}

 I'm quite fond of using wildcard types wherever possible, and I'd hate to
 lose that due to this issue.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:16>
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