[GHC] #2448: Type equality constraint not propagated to superclass

[GHC]

#2448: Type equality constraint not propagated to superclass
---------------------------------------------------------+------------------
    Reporter:  conal                                     |       Owner:          
        Type:  bug                                       |      Status:  new     
    Priority:  normal                                    |   Component:  Compiler
     Version:  6.9                                       |    Severity:  normal  
    Keywords:  type families, type equality constraints  |    Testcase:          
Architecture:  Unknown                                   |          Os:  Unknown 
---------------------------------------------------------+------------------
 The code:


 {{{
   {-# LANGUAGE TypeFamilies, UndecidableInstances #-}

   -- Demonstrates a bug in propagating type equality constraints

   class VectorSpace v where
     type Scalar v :: *

   class VectorSpace v => InnerSpace v

   instance (VectorSpace u,VectorSpace v, Scalar u ~ Scalar v) =>
 VectorSpace (u,v) where
     type Scalar (u,v) = Scalar u

   instance (InnerSpace u,InnerSpace v, Scalar u ~ Scalar v) => InnerSpace
 (u,v)
 }}}

 In ghc 6.9.20080622, the last line yields

 {{{
   Data\TypeEqBug.hs:20:0:
       Couldn't match expected type `Scalar v'
              against inferred type `Scalar u'
       When checking the super-classes of an instance declaration
       In the instance declaration for `InnerSpace (u, v)'
 }}}

 I'd expect the type equality constraint in the {{{InnerSpace}}} instance
 to get propagated to the parent class.

 By the way, without the {{{UndecidableInstances}}} above, we get:

 {{{
     Constraint is no smaller than the instance head
       in the constraint: Scalar u ~ Scalar v
     (Use -XUndecidableInstances to permit this)
     In the instance declaration for `VectorSpace (u, v)'
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2448>
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