[GHC] #11272: Overloaded state-monadic function is not specialised

[GHC]

#11272: Overloaded state-monadic function is not specialised
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           Reporter:  NickSmallbone  |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I have a simple typeclass-polymorphic function which fails to specialise.
 Here is module A which defines the function `overloaded`:

 {{{#!hs
 module A where

 import Control.Monad.Trans.State
 import Control.Monad

 overloaded :: Ord a => a -> a -> State () ()
 overloaded x y = do
   () <- get
   when (x <= y) (overloaded y x)
 }}}

 In module B I use `overloaded` on `Int`s:

 {{{#!hs
 module B where

 import A
 import Control.Monad.Trans.State

 specialised :: Int -> Int -> ()
 specialised x y = execState (A.overloaded x y) ()
 }}}

 Unfortunately the generated code is not specialised but passes an `Ord`
 dictionary around. It doesn't make any difference if I mark `overloaded`
 as `INLINEABLE` or not.

 In the core file, `overloaded` has been worker-wrapper transformed but the
 worker is marked `INLINEABLE[0]` - so I'm not sure why it's not being
 specialised. Curiously, if I make `overloaded` be a normal function
 instead of one in the state monad, or if I replace `() <- get` with simply
 `get`, specialisation goes through fine.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11272>
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