[GHC] #11734: coercionKind and coercionKindRole do potentially inefficient repeated substitutions

[GHC]

#11734: coercionKind and coercionKindRole do potentially inefficient repeated
substitutions
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           Reporter:  niteria        |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 As pointed out by @simonpj in phab:D2024:

 > I hate the idea of substituting for a single type variable at a time.
 Its a recipe for non-linear behaviour. Maybe we should gather the foralls
 and the type args and try to do it all at once?
 >
 > Anyway, that's not the fault of this patch, although this patch may make
 it worse. Because when substituting into a monotype, the inscope set is
 not used; but if we do it one at a time we substitute into a`ForallCo` so
 we do inspect the inscope set.
 >
 > See Type.piResultTys.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11734>
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