Overlapping typeclass instance selection depends on the optimisation level
A file A defines a typeclass, and gives an instance for all types a, and exports a function relying on said typeclass. A file B defines a data type, makes it a specific OVERLAPPING
instance of that class, and uses the function defined in A. Which instance ends up being picked for B depends on the optimisation level those files are compiled with.
- *Minimal test case**
//A.hs//
{-# LANGUAGE FlexibleInstances #-}
{-# OPTIONS_GHC -fno-warn-simplifiable-class-constraints #-}
module A where
import Data.Maybe
class A a where
someValue :: a -> Maybe Int
instance A a where
someValue = const Nothing
getInt :: A a => a -> Int
getInt x = fromMaybe 0 $ someValue x
//B.hs//
module B where
import A
data B = B Int
instance {-# OVERLAPPING #-} A B where
someValue (B x) = Just x
getBInt :: Int
getBInt = getInt $ B 42
//Main.hs//
import B
main :: IO ()
main = putStrLn $ "B: " ++ show getBInt
To reproduce:
$ ghc -O0 -fforce-recomp Main.hs && ./Main
[1 of 3] Compiling A ( A.hs, A.o )
[2 of 3] Compiling B ( B.hs, B.o )
[3 of 3] Compiling Main ( Main.hs, Main.o )
Linking Main ...
B: 42
$ ghc -O2 -fforce-recomp Main.hs && ./Main
[1 of 3] Compiling A ( A.hs, A.o )
[2 of 3] Compiling B ( B.hs, B.o )
[3 of 3] Compiling Main ( Main.hs, Main.o )
Linking Main ...
B: 0
The fix introduced to fix #14434 (closed) instructs the "short-cut solver" to not automatically choose a matching instance if it marked as INCOHERENT
or OVERLAPPABLE
, but in this case the instance is not marked in any way. This might be the source of the bug?
Additionally, whatever the optimisation level, ghc emits a warning about the A a =>
class constraint being simplifiable; but if it is removed, then the program prints "0" in both cases.
Trac metadata
Trac field | Value |
---|---|
Version | 8.4.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |