[GHC] #11282: Injectivity annotation fails in the presence of higher-rank use of constraint family

[GHC]

#11282: Injectivity annotation fails in the presence of higher-rank use of
constraint family
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        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
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 simonpj):

 Yes, this is unfortunate.  Here's a bit more detail.

 The ambiguity check does a subsumption check
 {{{
   forall a. (G a => F a) -> ()   <=    forall b. (G b => F b) -> ()
 }}}
 So we skolemise `b` and instantiate `b` to `beta`:
 {{{
   forall a. (   (G a => F a) -> ()   <=    (G beta => F beta) -> ()  )
 }}}
 Now we do co/contravariance (see `Note [Co/contra-variance of subsumption
 checking]` in TcUnify) to get
 {{{
   forall a.  (G beta => F beta)   <=    (G a -> F a)
 }}}
 Now we skolemise and instantiate again (in this case there are no foralls,
 but we still get an implication constraint:
 {{{
   forall a.   (forall. G beta =>   (F a ~ F beta))
 }}}
 Now as you point out beta is untouchable, and we can't float out the
 equality because `G beta` might have equalities in it.  Alas.

 I'm not sure what to do about this.  But that's what's happening.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11282#comment:2>
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