Type synonyms in class associated types behave strangely
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}
type Syn a = Associated a
class Eq (Associated a) => Foo a where
type Associated a :: *
foo :: a -> Syn a -> Bool
instance Foo () where
type Associated () = Int
foo _ x = x == x
In 7.6.3 and 7.8.1-rc2, this file compiles. In 7.8.1 release, GHC produces this error:
No instance for (Eq (Syn ())) arising from a use of ‘==’
In the expression: x == x
In an equation for ‘foo’: foo _ x = x == x
In the instance declaration for ‘Foo ()’
even though there is an Eq instance for Associated (), and Syn is just a type synonym.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |