[GHC] #8155: Defaulting bug or unfortunate error message with closed type families

[GHC]

#8155: Defaulting bug or unfortunate error message with closed type families
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        Reporter:  nh2               |            Owner:
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:  invalid           |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by nh2):

 Replying to [comment:1 goldfire]:
 > This may be an infelicity in defaulting (which I'm not terribly familiar
 with)
 >
 > [...]
 >
 > I will close this ticket, but do please reopen if you can characterize
 the bug as a specific infelicity in the defaulting mechanism.

 I know too little about how defaulting works, we would need somebody who
 understands that.

 > This is all very subtle and can be unexpected, but after lots and lots
 of thinking, Simon PJ, Dimitrios V, and I could not come up with a better
 way here

 I am not saying that I find this behaviour of closed type families wrong
 or so.

 Is it not possible to change the generated error message to give a hint
 why this is rejected, e.g. based on what you explained here?

 I understand your reasoning here, but had I not seen your example here and
 would I encounter this message in GHC with the current error message,
 chances are high I'd think it's a bug.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8155#comment:2>
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