#7123: Dictionary method should be applicable even in existential context
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Reporter: heisenbug | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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{{{
{-# LANGUAGE GADTs, TypeFamilies #-}
class Foo a where
type Bar a
baz :: Bar a -> Int
instance Foo Int where
type Bar Int = (Int, Int)
baz (i, _) = i
data Quux where
Quux :: Foo a => a -> Bar a -> Quux
q = Quux (42 :: Int) (1, 2)
i = case q of Quux _ b -> baz b
}}}
repro.hs:18:31:
Could not deduce (Bar a0 ~ Bar a)
from the context (Foo a)
bound by a pattern with constructor
Quux :: forall a. Foo a => a -> Bar a -> Quux,
in a case alternative
at repro.hs:18:15-22
NB: `Bar' is a type function, and may not be injective
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
In the first argument of `baz', namely `b'
In the expression: baz b
In a case alternative: Quux _ b -> baz b
Failed, modules loaded: none.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7123>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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