[GHC] #5751: code works in 7.0 but hangs in 7.2/7.4 due to changes in type checker (most likely)

[GHC]

#5751: code works in 7.0 but hangs in 7.2/7.4 due to changes in type checker (most
likely)
-----------------------------------------+----------------------------------
 Reporter:  JeremyShaw                   |          Owner:                
     Type:  bug                          |         Status:  new           
 Priority:  normal                       |      Component:  Compiler      
  Version:  7.2.1                        |       Keywords:                
       Os:  Linux                        |   Architecture:  x86_64 (amd64)
  Failure:  Incorrect result at runtime  |       Testcase:                
Blockedby:                               |       Blocking:                
  Related:                               |  
-----------------------------------------+----------------------------------
 I have a function with the lovely type signature:

 {{{
 homePage :: (Happstack m,
              MonadRoute m,
              MonadUser m,
              HasAppConf m,
              ToMessage (HSX.XML m),
              MonadRender m,
              Ontology.MkURL (URL m),
              EmbedAsAttr m (Attr String (URL m))) =>
             m Response
 }}}

 The code works fine in 7.0 but hangs (at runtime) under 7.2 and 7.4rc1.

 If I change that type signature to the more specific type:

 {{{
 homePage :: AppPart' Response
 }}}

 Then the code starts working again.

 This bug reminds me a lot of these previously closed bugs:

 #4809 #3731

 I will follow up with an isolated test case in the next couple days. It
 will likely take some time to tease out the relevant parts of the code
 into something more manageable.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5751>
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