Re: [GHC] #13804: MonoLocalBinds/RankNTypes type inference regression in GHC 8.2

#13804: MonoLocalBinds/RankNTypes type inference regression in GHC 8.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: Component: Compiler (Type | Version: 8.2.1-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: #11698 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"dc8e6861dc5586a8222484afc3bd26c432e2d69c/ghc" dc8e6861/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="dc8e6861dc5586a8222484afc3bd26c432e2d69c" Fix the treatment of 'closed' definitions The IdBindingInfo field of ATcId serves two purposes - to control generalisation when we have -XMonoLocalBinds - to check for floatability when dealing with (static e) These are related, but not the same, and they'd becomme confused. Trac #13804 showed this up via an example like this: f periph = let sr :: forall a. [a] -> [a] sr = if periph then reverse else id sr2 = sr -- The question: is sr2 generalised? -- It should be, because sr has a type sig -- even though it has periph free in (sr2 [True], sr2 "c") Here sr2 should be generalised, despite the free var 'periph' in 'sr' because 'sr' has a closed type signature. I documented all this very carefully this time, in TcRnTypes: Note [Meaning of IdBindingInfo] Note [Bindings with closed types: ClosedTypeId] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13804#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler