[GHC] #11369: Suppress redundant-constraint warnings in case of empty classes

[GHC]

#11369: Suppress redundant-constraint warnings in case of empty classes
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           Reporter:  hvr            |             Owner:  simonpj
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The technique described in

 http://www.well-typed.com/blog/2015/07/checked-exceptions/

 makes use of a class without any methods, i.e.

 {{{#!hs
 class Throws e
 }}}

 However, this by definition results in redundant constraints, e.g.

 {{{#!hs
 readFoo :: Throws IOError => FilePath -> IO Foo
 readFoo fn = {- ...  -}
 }}}

 which GHC 8.0 warns about.

 I propose to suppress warnings in case a class which has no methods is
 used as seemingly redundant constraint.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11369>
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