[GHC] #7398: RULES don't apply to a newtype constructor

[GHC]

#7398: RULES don't apply to a newtype constructor
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        Reporter:  shachaf           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  7.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #6082, #10418,    |  Differential Rev(s):
  #13290                             |
       Wiki Page:                    |
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Comment (by nomeata):

 >  The alternative is to try to make the rule work after inlining MkT, by
 being clever about casts. That might be possible.

 We somewhat do this already: We have
 {{{
 {-# RULES "map/coerce" [1] map coerce = coerce #-}
 }}}
 which fires in all these cases:
 {{{
 foo :: [Int] -> [Int]
 foo = map id
 fooAge :: [Int] -> [Age]
 fooAge = map Age
 fooCoerce :: [Int] -> [Age]
 fooCoerce = map coerce
 fooUnsafeCoerce :: [Int] -> [Age]
 fooUnsafeCoerce = map unsafeCoerce
 }}}
 (this is `testsuite/tests/simplCore/should_run/T2110.hs`).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7398#comment:16>
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