[GHC] #7135: Data.Bits can still have default implementations for testBit, bit, and popCount by using -XDefaultSignatures

[GHC]

#7135: Data.Bits can still have default implementations for testBit, bit, and
popCount by using -XDefaultSignatures
-------------------------------+--------------------------------------------
  Reporter:  joeyadams         |          Owner:                  
      Type:  feature request   |         Status:  closed          
  Priority:  normal            |      Milestone:  7.6.1           
 Component:  libraries/base    |        Version:  7.7             
Resolution:  wontfix           |       Keywords:                  
        Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown      |     Difficulty:  Unknown         
  Testcase:                    |      Blockedby:                  
  Blocking:                    |        Related:                  
-------------------------------+--------------------------------------------

Comment(by jwlato):

 I would request that you reconsider this decision. popCount was only added
 to the Bits class in base-4.5, which means that now Bits implementors are
 stuck in the following undesirable position:

 1. omit popCount, which may lead to runtime crashes if popCount is used in
 programs compiled with ghc-7.6 2. define popcount and drop support for ghc
 < 7.4 / base < 4.5\ 3. use CPP or cabal trickery to support ghc-7.2,
 ghc-7.4, and ghc-7.6

 For users who wish to support the three most recent ghc versions, none of
 these are good options.

 Upon reading the thread in libraries, I don't believe that anyone realized
 this issue at the time.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7135#comment:4>
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