[GHC] #5168: Inaccessible branches not detected

[GHC]

#5168: Inaccessible branches not detected
---------------------------------+------------------------------------------
    Reporter:  simonpj           |        Owner:              
        Type:  bug               |       Status:  new         
    Priority:  normal            |    Milestone:              
   Component:  Compiler          |      Version:  7.0.3       
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
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 C Rodrigues (red5_2 at hotmail.com) says: I was experimenting with using
 GADTs for subtyping when I found something interesting.  Hopefully someone
 can satisfy my curiosity.

 Here are two equivalent GADTs.  My understanding was that GHC would
 translate `Foo` and `Bar` into isomorphic data types.  However, GHC 6.12.3
 generates better code for `fooName` than for `barName`.  In `fooName`,
 there is no pattern match against `FooExtra`.  In `barName`, there is a
 pattern match against `BarExtra`.  What makes these data types different?

 {{{
 data Tag
 data TagExtra

 --------

 data Foo a where
   Foo :: String -> Foo a
   FooExtra :: IORef String -> Foo TagExtra

 -- The cmm code for fooName does not match against 'FooExtra'
 fooName :: Foo Tag -> String
 fooName (Foo s) = s

 --------

 data Bar a where
   Bar :: String -> Bar a
   BarExtra :: a ~ TagExtra => IORef String -> Bar a

 -- The cmm code for barName will try to pattern-match against 'BarExtra'
 barName :: Bar Tag -> String
 barName (Bar s) = s
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5168>
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