#2854: Extended defaulting rules not used for super classes
-----------------------------+----------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Consider the following module
{{{
{-# LANGUAGE ExtendedDefaultRules #-}
module Bug4 where
class (Num a) => C a where
f :: a -> a
instance C Integer where
f = (+1)
x = show (f 5)
}}}
The type of x is
{{{
x :: (C a) => String
}}}
and since a is ambiguous the defaulting rules should be used. Since Num
is a superclass of C the type variable should be defaulted to Integer.
But instead we get an error.
Removing the Num superclass of C makes it work, since the type of x is
{{{
x ::: (Num a, C a) => String
}}}
and the defaulting kicks in as expected.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2854>
GHC <http://www.haskell.org/ghc/>
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