#11480: UndecidableSuperClasses causes the compiler to spin with UndecidableInstances -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Keywords: PolyKinds, Resolution: | UndecidableSuperClasses Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10318 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"ff21795a0b9253e811a45626d5686e981ed07f82/ghc" ff21795a/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="ff21795a0b9253e811a45626d5686e981ed07f82" Special-case implicit params in superclass expansion This issue came up in Trac #11480, and is documented in Note [When superclasses help] in TcRnTypes. We were getting a spurious warning T11480.hs:1:1: warning: solveWanteds: too many iterations (limit = 4) The fix is easy. A bit of refactoring along the way. The original bug report in Trac #11480 appears to work fine in HEAD and the 8.0 branch but I added a regression test in this commit as well. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11480#comment:8> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler