[GHC] #14961: QuantifiedConstraints: introducing classes through equality constraints fails

[GHC]

#14961: QuantifiedConstraints: introducing classes through equality constraints
fails
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        Reporter:  mrkgnao           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14860            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 You want GHC to ''infer'' a type with a quantified constraint.  In general
 that is hard, and quite probably not what you wanted, so GHC doesn't even
 try.

 In short: no inferred type has a quantified constraint, only declared
 types.

 In GHCi, `:type <expression>` infers the type of `<expression>` so you
 have the same issue.  On the other hand `:info <identifier>` will work
 just fine.

 Does that make sense?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14961#comment:6>
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