#15805: Bug in anyRewritableTyVar -------------------------------------+------------------------------------- Reporter: ningning | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #14363 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The current implementation of `TcType.anyRewritableTyVar` has one wrong case: {{{#!hs go_tc ReprEq bvs tc tys = foldr ((&&) . go_arg bvs) False $ (tyConRolesRepresentational tc `zip` tys) }}} Here `foldr` uses `&&` in the function and a `False` as the initial value, which causes this case to always return `False` no matter what the inputs are. This bug is introduced in [changeset:"3acd6164fea6d4d5d87521a291455a18c9c9a8ee/ghc" 3acd616/ghc], which is for fixing #14363. The correct implementation (I think) should use `||` instead of `&&`: {{{#!hs go_tc ReprEq bvs tc tys = foldr ((||) . go_arg bvs) False $ (tyConRolesRepresentational tc `zip` tys) -- or just go_tc ReprEq bvs tc tys = any (go_arg bvs) (tyConRolesRepresentational tc `zip` tys) }}} We presume that there might be some program which loops because of this bug. Patch incoming. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15805 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler