[GHC] #12959: GHC doesn't warn about missing implementations for class methods beginning with an underscore

[GHC]

#12959: GHC doesn't warn about missing implementations for class methods beginning
with an underscore
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by RyanGlScott:

@@ -26,1 +26,1 @@
- Bug.hs:7:10: warning: [-Wmissing-methods]
+ Warn.hs:7:10: warning: [-Wmissing-methods]

New description:

 This code compiles without any warnings!

 {{{#!hs
 {-# OPTIONS_GHC -Wmissing-methods #-}
 module Bug where

 class Foo a where
   _Bar :: a -> Int

 instance Foo Int
 }}}

 This code, however, //does// emit a warning:

 {{{#!hs
 {-# OPTIONS_GHC -Wmissing-methods #-}
 module Warns where

 class Foo a where
   bBar :: a -> Int

 instance Foo Int
 }}}

 {{{
 Warn.hs:7:10: warning: [-Wmissing-methods]
     • No explicit implementation for
         ‘bBar’
     • In the instance declaration for ‘Foo Int’
 }}}

 The only difference is in the name of the class method of `Foo`. Namely,
 when a class method is prefixed with an underscore, GHC seems to look the
 other way when checking if any instances are missing an implementation of
 it.

 I'd argue that this behavior is incorrect, since I can't envision any
 scenario in which you'd want the compiler //not// to warn you about
 missing method implementations (as opposed to, say, not warning about an
 unused record selector).

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12959#comment:1>
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