[ ghc-Bugs-1206891 ] unjustified deduction error

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Bugs item #1206891, was opened at 2005-05-23 01:13
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Category: Compiler (Type checker)
Group: 6.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: unjustified deduction error

Initial Comment:
(also sent to glasgow-haskell-users) maeder at tzi.de

the following (reduced) example used to go through with
ghc-6.2.2 but fails with ghc-6.4. 

This seems to be a - low priority - bug. (hugs accepts
this module.)
I compile with:

ghc -fglasgow-exts Context.hs


module Context where

class Language a
class Language a => Logic a b | a -> b
class (Language a, Logic b c, Logic d e)
    => Comorph a b c d e | a -> b, a -> d

instance (Comorph a1 b1 c1 d1 e1, Comorph a2 b2 c2 d2 e2)
    => Language (a1, a2)

instance (Comorph a1 b1 c1 d1 e1, Comorph a2 b2 c2 d2 e2)
    => Comorph (a1, a2) b1 c1 d2 e2

-- end of module

ghc-6.4 (or ghc-6.4.1) complains with:

Context.hs:11:0:
    Could not deduce (Comorph a2 b2 c21 d2 e21, Comorph
a1 b1 c11 d1 e11)
      from the context (Comorph a1 b1 c1 d1 e1, Comorph
a2 b2 c2 d2 e2)
      arising from the superclasses of an instance
declaration at
Context.hs:11:0
    Probable fix:
      add (Comorph a2 b2 c21 d2 e21, Comorph a1 b1 c11
d1 e11)
      to the instance declaration superclass context
    In the instance declaration for `Comorph (a1, a2)
b1 c1 d2 e2'


If I replace the first instance with
"instance (Language a1, Language a2) => Language (a1, a2)"
then ghc-6.4 is happy.

Cheers Christian

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