Unexpected 'Overlapping instances' occurs on debian wheezy (ghc-7.4.1) about doctest

[sol]

Most likely a GHCi bug. But we should investigate. Can you try to reproduce with GHCi only?

[notogawa]

I try it and reproduce the issue.

On ghc-7.4.1

$ ghci -package-conf .cabal-sandbox/x86_64-linux-ghc-7.4.1-packages.conf.d TestTarget.hs
*TestTarget> T == T
True
*TestTarget> import Test.QuickCheck
*TestTarget Test.QuickCheck> T == T
True
*TestTarget Test.QuickCheck> instance Arbitrary T where arbitrary = return T
*TestTarget Test.QuickCheck> T == T

<interactive>:10:3:
    Overlapping instances for Eq T
      arising from a use of `=='
    Matching instances:
      instance Eq T -- Defined at TestTarget.hs:14:21
      instance Eq T -- Defined at TestTarget.hs:14:21
    In the expression: T == T
    In an equation for `it': it = T == T

On ghc-7.6.3

$ ghci -package-db .cabal-sandbox/x86_64-linux-ghc-7.6.3-packages.conf.d TestTarget.hs
*TestTarget> T == T
True
*TestTarget> import Test.QuickCheck
*TestTarget Test.QuickCheck> T == T
True
*TestTarget Test.QuickCheck> instance Arbitrary T where arbitrary = return T
*TestTarget Test.QuickCheck> T == T
True

And I found this ghc issue.

Is there any workaround ? Defining Arbitrary instances in the source is the only way to use doctest with QuickCheck ?

[sol]

I guess this is fixed in GHC 7.4.2. The solution is to not use the version of GHC that comes with Debian, but install your own. You can try the binary builds from http://www.haskell.org/ghc/ (they may or may not work, doing generic binary builds is hard). If you are on wheezy, you can also use http://typeful.net/ghc/.

[notogawa]

Thanks.