#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 RyanGlScott): Specifically, the issue seems to pertain to type equalities in default signatures in typeclasses. Here is the failing code, reduced to reproducible examples: From [http://hackage.haskell.org/package/microlens-0.4.2.0/docs/src/Lens- Micro-Internal.html#line-184 microlens]: {{{#!hs {-# LANGUAGE DefaultSignatures #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} type Traversal s t a b = forall f. Applicative f => (a -> f b) -> s -> f t class Each s t a b | s -> a, t -> b, s b -> t, t a -> s where each :: Traversal s t a b default each :: (Traversable g, s ~ g a, t ~ g b) => Traversal s t a b each = traverse }}} From [http://hackage.haskell.org/package/free-4.12.4/docs/src/Control- Monad-Free-Class.html#line-106 free]: {{{#!hs {-# LANGUAGE DefaultSignatures #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} class MonadTrans t where lift :: Monad m => m a -> t m a class Monad m => MonadFree f m | m -> f where wrap :: f (m a) -> m a default wrap :: (m ~ t n, MonadTrans t, MonadFree f n, Functor f) => f (m a) -> m a wrap = join . lift . wrap . fmap return }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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: 8.0.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by hvr): Just to be clear, afaik this doesn't occur with 8.0.1-rc2, but rather with a more recent 8.0.01 snapshot (as described in the description). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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: 8.0.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by hvr): I've temporarily reverted e1631b3b58b7440d3d5a8bf72f1490df635792fb in the `ghc-8.0` branch ''only'', via 881b6ccf5c1dbc09d1d16b1b4643e3dec9387047. I'm not reverting this in GHC HEAD yet, waiting for Richard to comment how we should proceed here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens` -------------------------------------+------------------------------------- Reporter: hvr | Owner: goldfire Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Aha. In a class decl we use a `TyVar` (not a skolem `TcTyVar`) for the class variable(s). (There is a good reason for this: in the final `Class` value we don't want any `TcTyVar`s.) Then in `TcFlatten.flatten_tyvar` we see {{{ flatten_tyvar tv | not (isTcTyVar tv) -- Happens when flatten under a (forall a. ty) = flatten_tyvar3 tv }}} This case catches that class variable. In the ambiguity check for the type sig for `each` we have a Given equality for the class variable. Then, because of the above test we fail to take advantage of the Given. (And the error message too is misleading.) Solution: delete the above equation, so that we treat `TyVar` and skolem `TcTyVar` the same. I'm validating now. The `TyVar`/`TcTyVar` story is not great. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens` -------------------------------------+------------------------------------- Reporter: hvr | Owner: simonpj Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"7496be5c0ab96bcc9ab70ab873aa561674b7789d/ghc" 7496be5c/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="7496be5c0ab96bcc9ab70ab873aa561674b7789d" Exclude TyVars from the constraint solver There is a general invariant that the constraint solver doesn't see TyVars, only TcTyVars. But when checking the generic-default signature of a class, we called checkValidType on the generic-default type, which had the class TyVar free. That in turn meant that it wasn't considered during flattening, which led to the error reported in Trac #11608. The fix is simple: call checkValidType on the /closed/ type. Easy. While I was at it, I added a bunch of ASSERTs about the TcTyVar invariant. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens` -------------------------------------+------------------------------------- Reporter: hvr | Owner: simonpj Type: bug | Status: merge Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | typecheck/should_compile/T11608 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => merge * testcase: => typecheck/should_compile/T11608 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens` -------------------------------------+------------------------------------- Reporter: hvr | Owner: simonpj Type: bug | Status: closed Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | typecheck/should_compile/T11608 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Hm. These `ASSERT`s used to be here, and I removed them because they were triggering on kind variables, which were not always `TcTyVar`s. However, if the `dependent` set of tests in the testsuite aren't failing here, then perhaps things improved from when I removed the `ASSERT`s. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens` -------------------------------------+------------------------------------- Reporter: hvr | Owner: simonpj Type: bug | Status: closed Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | typecheck/should_compile/T11608 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by thomie): Travis is reporting this at the moment: {{{ Unexpected failures: polykinds MonoidsTF [exit code non-0] (normal) polykinds T11480b [exit code non-0] (normal) polykinds T11523 [exit code non-0] (normal) th T3100 [exit code non-0] (normal) typecheck/should_compile T3692 [exit code non-0] (normal) typecheck/should_fail T3592 [stderr mismatch] (normal) typecheck/should_fail TcCoercibleFail [stderr mismatch] (normal) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens` -------------------------------------+------------------------------------- Reporter: hvr | Owner: simonpj Type: bug | Status: merge Priority: highest | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | typecheck/should_compile/T11608 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: closed => merge Comment: Here's the patch for the other three {{{ commit 3c29c770be7a8c7268dcb8d8624853428aa42071 Author: Simon Peyton Jones Date: Mon Feb 29 14:12:28 2016 +0000 Do not check synonym RHS for ambiguity With this patch we no longer check the RHS of a type synonym declaration for ambiguity. It only affects type synonyms with foralls on the RHS (which are rare in the first place), and it's arguably over-aggressive to check them for ambiguity. See TcValidity Note [When we don't check for ambiguity] This fixes the ASSERT failures in th T3100 typecheck/should_compile T3692 typecheck/should_fail T3592 }}} Just about worth merging. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11608#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler