[GHC] #15840: Inline data constructor wrappers very late

[GHC]

#15840: Inline data constructor wrappers very late
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        Reporter:  aspiwack          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.7
      Resolution:                    |             Keywords:
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 aspiwack):

 Here is an example program which changes behaviour depending on this
 ticket:

 {{{#!haskell
 data T = MkT !Bool

 f :: T -> Bool
 f _ = False
 {-# NOINLINE f #-}

 {-# RULES "non-det" [1] forall x. f (MkT x) = x #-}

 main :: IO ()
 main = print (f (MkT True))
 }}}

 Since `MkT` has a strict field, it has a wrapper. The rule triggers only
 if the wrapper hasn't been inlined, however, wrappers are inlined (current
 GHC) in phase 2, and the rules is only active in phase 1 (and later). So
 the rule never fires, and the program prints `False`.

 With the proposed patch, the wrapper is not inlined before phase 0, so the
 rule can fire and the program prints `False`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15840#comment:2>
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