[GHC] #14292: Coercing between constraints of newtypes

[GHC]

#14292: Coercing between constraints of newtypes
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
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 RyanGlScott):

 Replying to [comment:3 AntC]:
 > `IncoherentInstances` is a module-wide setting; and nowadays deprecated
 in favour of the more surgical `{-# INCOHERENT #-}` pragma on the
 instance.
 >
 > Does that also apply for the role mungling?

 As far as I know, you do need to enable the `IncoherentInstances`
 extension proper in order to assign non-nominal roles to class parameters,
 as the `{-# INCOHERENT #-}` pragma only works for instance declarations,
 not class declarations. (This might be partly why you don't get a
 deprecation warning when enabling `IncoherentInstances`, but you do with
 `OverlappingInstances`, which doesn't have a dual purpose like
 `IncoherentInstances` does.)

 > How does the `{-# INCOHERENT #-}` pragma go for `StandaloneDeriving` or
 `GeneralizedNewtypeDeriving`? It seems to be accepted by GHC OK, does it
 have the right effect? (It's hard to put together a test case.)

 I'm not sure I understand the question. Derived instances never use `{-#
 INCOHERENT #-}` (or even `{-# OVERLAPS #-}`/`{-# OVERLAPPING #-}`/`{-#
 OVERLAPPABLE #-}`, for that matter) in their emitted code, if that's what
 you're wondering.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14292#comment:4>
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