[GHC] #11735: Optimize coercionKind

[GHC]

#11735: Optimize coercionKind
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 It's nice that `OptCoercion` for `NthCo` gets simpler when we have the
 role cached.

 > I allowed a more permissive role in the NthCo

 OK, but

 * I didn't see a change to `mkSubCo` that would take advantage of this (by
 changing the role rather than wrapping in `SubCo`)

 * The comment on the constructor `NthCo` should state the invariant.
 Given `(NthCo r n co)` we require that `lteRole r (nthRole (coercionRole
 co) n))` or something like that.

 * Why does't the same flexibility apply for the other coercions that cache
 roles: `FunCo`, `Refl`, `TyConAppCo`, `UnivCo`?   We should state their
 invariants too...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11735#comment:61>
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