Common Sense for Type Classes
GHC erroneously reports that there is "No instance of C". But, in fact, there is an instance, C Int Char
; furthermore, it is the only instance. So GHC should use it!
{-# LANGUAGE MultiParamTypeClasses #-}
module T4921 where
class C a b where
f :: (a,b)
instance C Int Char where
f = undefined
x = fst f
y = fst f :: Int
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |