[GHC] #4361: Typechecker regression

[GHC]

#4361: Typechecker regression
----------------------------------------+-----------------------------------
    Reporter:  igloo                    |        Owner:  simonpj     
        Type:  bug                      |       Status:  new         
    Priority:  high                     |    Milestone:  7.0.1       
   Component:  Compiler (Type checker)  |      Version:  7.1         
    Keywords:                           |     Testcase:              
   Blockedby:                           |   Difficulty:              
          Os:  Unknown/Multiple         |     Blocking:              
Architecture:  Unknown/Multiple         |      Failure:  None/Unknown
----------------------------------------+-----------------------------------
 Here's the essence of the regression DoCon showed up, reported here:
 http://www.haskell.org/pipermail/glasgow-haskell-
 bugs/2010-September/025511.html

 {{{
 {-# LANGUAGE FlexibleContexts #-}

 module Pol3_ (moduloBasisx) where

 class CommutativeRing a
 class CommutativeRing a => LinSolvRing a
 class LinSolvRing a => EuclideanRing a

 instance EuclideanRing a => LinSolvRing (Pol a)
 instance CommutativeRing a => CommutativeRing (Pol a)

 data Pol a = MkPol

 upLinSolvRing :: LinSolvRing a => a -> ()
 upLinSolvRing = undefined

 moduloBasisx :: (LinSolvRing (Pol a), CommutativeRing a) => Pol a -> ()
 moduloBasisx p = let x = upLinSolvRing p
                  in ()
 }}}

 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 6.12.2
 $ ghc --make Pol3_
 [1 of 1] Compiling Pol3_            ( Pol3_.hs, Pol3_.o )
 $
 }}}

 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.0.0.20100924
 $ ghc --make Pol3_
 [1 of 1] Compiling Pol3_            ( Pol3_.hs, Pol3_.o )

 Pol3_.hs:19:26:
     Could not deduce (EuclideanRing a)
       from the context (LinSolvRing (Pol a), CommutativeRing a)
       arising from a use of `upLinSolvRing'
     Possible fix:
       add (EuclideanRing a) to the context of
         the type signature for `moduloBasisx'
     In the expression: upLinSolvRing p
     In an equation for `x': x = upLinSolvRing p
     In the expression: let x = upLinSolvRing p in ()
 $
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4361>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler