[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 RyanGlScott):

 bgamari, your suspicions have been confirmed:

 {{{
 $ ghc2/inplace/bin/ghc-stage2 -O2 -fforce-recomp -DDEBUG Bug.hs
 [1 of 1] Compiling Bug2             ( Bug.hs, Bug.o )
 WARNING: file compiler/specialise/SpecConstr.hs, line 2078
   SpecConstr: bad covars: [sg_s2Ba]
   $wslowGo_s2A8 (-# ds_X2vv 1#)
                 (wild_Xi
                  `cast` (Sym (N:Foo[0] <(Int, Int)>_R)
                          :: ((Int, Int) :: *) ~R# (Foo (Int, Int) :: *)))
                 w_s2A3
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14936#comment:9>
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