#7861: deferred type error with rankNTypes -------------------------------------+------------------------------------- Reporter: guest | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Resolution: wontfix | Keywords: Operating System: Linux | Architecture: x86_64 (amd64) Type of failure: Compile-time | Test Case: crash | typecheck/should_run/T7861 Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"4c96e7cf56fc67ad09efaf8c5de1c8d7a0f5cedd/ghc" 4c96e7cf/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="4c96e7cf56fc67ad09efaf8c5de1c8d7a0f5cedd" Testsuite: add ImpredicativeTypes to T7861 (#7861) The test was failing with: T7861: T7861.hs:15:13: Cannot instantiate unification variable ‘t0’ with a type involving foralls: A a0 -> a0 GHC doesn't yet support impredicative polymorphism In the first argument of ‘seq’, namely ‘f’ In a stmt of a 'do' block: f `seq` print "Hello 2" It requires ImpredicativeTypes, at least since 7.8, because we instantiate seq's type (c->d->d) with f's type (c:= (forall b. a) -> a), which is polymorphic (it has foralls). I simplified the test a bit by removing the type synonym, and verified that ghc-7.6.3 still panics on this test. Reviewers: simonpj, austin, bgamari Reviewed By: bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1080 GHC Trac Issues: #7861 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7861#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler