Typechecker regression in GHC 8.0.2 involving DefaultSignatures

This code compiles without issue on GHC 8.0.1 and earlier, but not with GHC 8.0.2 or HEAD. This was adapted from the monad-logger library (which fails to build with GHC 8.0.2 and HEAD due to the same issue):

{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE GADTs #-}
module MonadLogger where

import           Control.Monad.IO.Class
import qualified Control.Monad.Trans.Class as Trans
import           Control.Monad.Trans.Identity

data Loc
data LogSource
data LogLevel
data LogStr

class ToLogStr msg
class Monad m => MonadLogger m
class (MonadLogger m, MonadIO m) => MonadLoggerIO m where
    askLoggerIO :: m (Loc -> LogSource -> LogLevel -> LogStr -> IO ())
    default askLoggerIO :: (Trans.MonadTrans t, MonadLogger (t m), MonadIO (t m))
                        => t m (Loc -> LogSource -> LogLevel -> LogStr -> IO ())
    askLoggerIO = Trans.lift askLoggerIO

instance MonadLogger m => MonadLogger (IdentityT m)
instance MonadLoggerIO m => MonadLoggerIO (IdentityT m)

On GHC HEAD, this fails with:

[1 of 1] Compiling MonadLogger      ( MonadLogger.hs, interpreted )

MonadLogger.hs:23:10: error:
    • Couldn't match type ‘m’ with ‘IdentityT m’
      ‘m’ is a rigid type variable bound by
        the instance declaration at MonadLogger.hs:23:10-55
      Expected type: IdentityT
                       m (Loc -> LogSource -> LogLevel -> LogStr -> IO ())
        Actual type: IdentityT
                       (IdentityT m) (Loc -> LogSource -> LogLevel -> LogStr -> IO ())
    • In the expression: MonadLogger.$dmaskLoggerIO @IdentityT m
      In an equation for ‘askLoggerIO’:
          askLoggerIO = MonadLogger.$dmaskLoggerIO @IdentityT m
      In the instance declaration for ‘MonadLoggerIO (IdentityT m)’
    • Relevant bindings include
        askLoggerIO :: IdentityT
                         m (Loc -> LogSource -> LogLevel -> LogStr -> IO ())
          (bound at MonadLogger.hs:23:10)

This stopped typechecking after d2958bd0 (i.e, #12220 (closed)).

As a workaround, you can change the default signature to:

    default askLoggerIO :: (Trans.MonadTrans t, MonadLoggerIO n, t n ~ m)
                         => t n (Loc -> LogSource -> LogLevel -> LogStr -> IO ())

And it'll work.

Any thoughts on this, Simon?

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonpj
Operating system
Architecture