[GHC] #15621: Error message involving type families points to wrong location

[GHC]

#15621: Error message involving type families points to wrong location
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
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 goldfire):

 This sniffs awfully like wanteds rewriting wanteds.

 I don't agree with the details in comment:3. You have constraints like
 `alpha ~ F Int`. But `CTyEqCan` documents "rhs ... has no top-level
 function". So these constraints look malformed. I would expect an
 intervening flatten metavar, which may change what's going on here.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15621#comment:4>
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