[GHC] #16070: Better inferred signatures

[GHC]

#16070: Better inferred signatures
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by gridaphobe):

 Replying to [comment:6 AntC]:
 > Note also a later comment on that github discussion that the `FieldType`
 constraint holding implies there's a `HasField` instance -- because
 `FieldType` is an Associated Type. Using that implication seems harder:
 Assoc Types are treated as sugar for top-level type families/not
 necessarily grounded in instances, which already gives trouble.

 I was also concerned about this, in particular what happens if we have a
 `type instance` for an associated type at the top-level? But it turns out
 that GHC does not allow that, all instances of associated types must be
 defined in the context of a class instance. So I think using the
 `FieldType => HasField` implication could work nicely.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16070#comment:7>
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