#4200: UndecidableInstances not needed for equality constraints
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.12.3
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Consider
{{{
instance (Op c, In c ~ Int) => Op (D2 c) where ...
}}}
Without `-XUndecidableInstaces` we get
{{{
undec.hs:21:0:
Constraint is no smaller than the instance head
in the constraint: In c ~ Int
(Use -XUndecidableInstances to permit this)
}}}
But the undecidable-instance check is only needed for class constraints I
think, so GHC is being over conservative. Let's fix this.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4200>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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