[GHC] #15052: DeriveAnyClass instances may skip TypeError constraints
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Wed Apr 18 17:16:09 UTC 2018
#15052: DeriveAnyClass instances may skip TypeError constraints
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Reporter: jcpetruzza | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone: 8.6.1
Component: Compiler | Version: 8.2.2
Resolution: | Keywords:
| CustomTypeErrors
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by jcpetruzza):
I don't disagree with anything you are saying.
The issue is that `deriving` is behaving differently from `instance` and
from `deriving instance`. Consider this hopefully more clear example:
{{{#!haskell
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}
module T2 where
import Data.Kind(Constraint)
import GHC.TypeLits(TypeError, ErrorMessage(..))
class D a where
f :: a
default f :: DeferError a => a
f = error "unreachable"
type family DeferError a :: Constraint where
DeferError a = TypeError ('Text "Boom")
data X
deriving D -- < -- ACCEPTED
data Y
instance D Y -- < -- REJECTED
data Z
deriving instance D Z -- < -- REJECTED
}}}
Do you aggree that either the three instances should be accepted here or
the three instances should be rejected?
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15052#comment:11>
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