Bug: instances from non-imported files used

[Neil Mitchell]

Hi,

With the following setup:

--
module A where
data A = A
--
module B where
import A
b = show A
--
module C where
import A
instance Show A where
   show A = "A"
--
import B
import C

main = print "hello"
--

:load B fails, complaining that there is no Show instance for A, which
is correct.

However, :load Main succeeds, even though the module B that it imports
is valid. It seems that the show instance of C is used in B, even
though the module B does not import (even transitively) C.

GHC fails with:
./B.hs:6:4:
    No instance for (Show A)
      arising from use of `show' at ./B.hs:6:4-7
    Probable fix: add an instance declaration for (Show A)
    In the definition of `b': b = show A

Using Hugs May 2006 on Windows.

Thanks

Neil