[GHC] #15453: Bug in opt_trans_rule in OptCoercion

[GHC]

#15453: Bug in opt_trans_rule in OptCoercion
-------------------------------------+-------------------------------------
        Reporter:  ningning          |                Owner:  ningning
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5018
       Wiki Page:                    |
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Comment (by Krzysztof Gogolewski <krz.gogolewski@…>):

 In [changeset:"11de4380c2f16f374c6e8fbacf8dce00376e7efb/ghc" 11de438/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="11de4380c2f16f374c6e8fbacf8dce00376e7efb"
 Fix #15453: bug in ForAllCo case in opt_trans_rule

 Summary:
 Given

 ```
 co1 = \/ tv1 : eta1. r1
 co2 = \/ tv2 : eta2. r2
 ```

 We would like to optimize `co1; co2` so we push transitivity inside
 forall.
 It should be

 ```
 \/tv1 : (eta1;eta2).  (r1; r2[tv2 |-> tv1 |> eta1])
 ```

 It is implemented in the ForAllCo case in opt_trans_rule in OptCoercion.
 However current implementation is not right:

 ```
 r2' = substCoWithUnchecked [tv2] [TyVarTy tv1] r2 -- ill-kinded!
 ```

 This patch corrects it to be

 ```
 r2' = substCoWithUnchecked [tv2] [mkCastTy (TyVarTy tv1) eta1] r2
 ```

 Test Plan: validate

 Reviewers: bgamari, goldfire, RyanGlScott

 Reviewed By: RyanGlScott

 Subscribers: rwbarton, thomie, carter

 GHC Trac Issues: #15453

 Differential Revision: https://phabricator.haskell.org/D5018
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15453#comment:7>
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