id summary reporter owner description type status priority milestone component version resolution keywords cc os architecture failure testcase blockedby blocking related differential wikipage
5736 Possible regression with functional dependencies magnus "The following program typechecks in GHC-7.0.4, but not in 6.12.3, 7.2.2 or 7.5.20111229:
{{{
{-# LANGUAGE FunctionalDependencies, MultiParamTypeClasses,
FlexibleInstances, UndecidableInstances, TypeFamilies #-}
class C a b | a -> b where
c :: a -> b
instance C b b where
c = undefined
instance C Bool b where
c = undefined
m :: Int
m = c True
}}}
7.5.20111229 says
{{{
Couldn't match type `Bool' with `Int'
When using functional dependencies to combine
C b b,
arising from the dependency `a -> b'
in the instance declaration at Test.hs:7:10
C Bool Int, arising from a use of `c' at Test.hs:14:5
In the expression: c True
In an equation for `m': m = c True
}}}
Was I just lucky that this worked in 7.0.4, or is it a true regression?
" bug closed normal Compiler (Type checker) 7.2.1 invalid Unknown/Multiple Unknown/Multiple None/Unknown