GHC 8 superclass chain constraint regression
It seems that GHC 8 regresses with regard to GHC 7.10, when it tries to satisfy the constraints, implied by instance context.
The following does not build on GHC 8.0.1 RC2, but does on 7.10.3:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
--{-# LANGUAGE UndecidableSuperClasses #-} -- this doesn't matter
{-# LANGUAGE UnicodeSyntax #-}
module Foo where
class Super a
class Super a ⇒ Left a
class Super a ⇒ Right a
instance (Left a) ⇒ Right athe error being:
• Could not deduce (Super a) arising from the superclasses of an instance declaration from the context: Left a bound by the instance declaration at repro.hs:9:10-27 Possible fix: add (Super a) to the context of the instance declaration • In the instance declaration for ‘Right a’
It also /roughly/ seems that https://phabricator.haskell.org/D1594 could possibly be related.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1-rc2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
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