[GHC] #14936: GHC 8.4 performance regressions when using newtypes

[GHC]

#14936: GHC 8.4 performance regressions when using newtypes
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        Reporter:  danilo2           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.4.2
       Component:  Compiler          |              Version:  8.4.1
      Resolution:                    |             Keywords:  SpecConstr
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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"5ab8094e4579c08973260c2d18599be0738526ec/ghc" 5ab8094/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="5ab8094e4579c08973260c2d18599be0738526ec"
 SpecConstr: accommodate casts in value arguments

 This commit:

   commit fb050a330ad202c1eb43038dc18cca2a5be26f4a
   Author: Simon Peyton Jones <simonpj at microsoft.com>
   Date:   Thu Oct 12 11:00:19 2017 +0100

   Do not bind coercion variables in SpecConstr rules

 arranged to reject any SpecConstr call pattern that mentioned
 a coercion in the pattern.

 There was a good reason for that
 -- see Note [SpecConstr and casts] --
 but I didn't realise how important it was to accept patterns
 that mention casts in /terms/.  Trac #14936 showed this up.

 This patch just narrows the restriction to discard only
 the cases where the coercion is mentioned only in types.
 Fortunately that was pretty easy to do.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14936#comment:10>
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