[GHC] #16115: Missing associated type instance not reported with error

[GHC]

#16115: Missing associated type instance not reported with error
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler (Type    |              Version:  8.6.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Yes, that's true.  But it's generally reasonable for errors to suppress
 warnings.  And the error isn't too bad.  The user thinks "Why doesn't
 `Item Nat` equal `()`?  Oh, I'd better look in `instance FromList Nat`,
 since `Item` is an associate type of `FromList`; and sure enough there is
 no `type instance`.

 So it's not too bad.  If we wanted to change, I can imagine various things
 we could do:

 * Never suppress warnings, even if there are errors.
 * Make it compulsory to give an instance for an associated type, if there
 is no default in the class.  That'd be a language change.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16115#comment:1>
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