#9739: GHC 7.8 chokes on recursive classes -------------------------------------+------------------------------------- Reporter: bitonic | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.9 (Type checker) | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Compile- | Related Tickets: time crash | Test Case: | typecheck/should_fail/T9739 | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by simonpj): * testcase: => typecheck/should_fail/T9739 Comment: The 3-class version still loops in HEAD. Reason is this: * `TcTyClsDcls.checkValidClass` is called on each class in turn. * It checks for superclass cycles, and then checks for ambiguity of the method types * The ambiguity check loops if there is a superclass cycle * The superclass cycle check was first, and aborted `checkValidClass` if it failed. * BUT we failed to notice that it would loop if there was a superclass cycle in ''another'' class in the group; and after each class we dust ourselves off and the next one, to get as many validity errors as possible. Solution was simple: make the ambiguity check happen only if there have been no errors so far. Patch coming Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9739#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler