#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens` -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by hvr): hackage:free-4.12.4 has a similiar sudden failure: {{{ Preprocessing library free-4.12.4... [ 1 of 16] Compiling Control.Monad.Free.TH ( src/Control/Monad/Free/TH.hs, dist/build/Control/Monad/Free/TH.o ) [ 2 of 16] Compiling Control.Monad.Free.Class ( src/Control/Monad/Free/Class.hs, dist/build/Control/Monad/Free/Class.o ) src/Control/Monad/Free/Class.hs:106:3: error: • Couldn't match type ‘m’ with ‘t0 n0’ ‘m’ is untouchable inside the constraints: (m ~ t n, MonadTrans t, MonadFree f n, Functor f) bound by the type signature for: wrap :: (m ~ t n, MonadTrans t, MonadFree f n, Functor f) => f (m a) -> m a at src/Control/Monad/Free/Class.hs:106:3-24 • In the ambiguity check for ‘wrap’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the class method: wrap :: forall (f :: * -> *) (m :: * -> *). MonadFree f m => forall a. f (m a) -> m a In the class declaration for ‘MonadFree’ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler