#11408: Delicate solve order -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"9308c736d43b92bf8634babf565048e66e071bd8/ghc" 9308c736/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="9308c736d43b92bf8634babf565048e66e071bd8" Fix a number of subtle solver bugs As a result of some other unrelated changes I found that IndTypesPerf was failing, and opened Trac #11408. There's a test in indexed-types/should-compile/T11408. The bug was that a type like forall t. (MT (UL t) (UR t) ~ t) => UL t -> UR t -> Int is in fact unambiguous, but it's a bit subtle to prove that it is unambiguous. In investigating, Dimitrios and I found several subtle bugs in the constraint solver, fixed by this patch * canRewrite was missing a Derived/Derived case. This was lost by accident in Richard's big kind-equality patch. * Interact.interactTyVarEq would discard [D] a ~ ty if there was a [W] a ~ ty in the inert set. But that is wrong because the former can rewrite things that the latter cannot. Fix: a new function eqCanDischarge * In TcSMonad.addInertEq, the process was outright wrong for a Given/Wanted in the (GWModel) case. We were adding a new Derived without kicking out things that it could rewrite. Now the code is simpler (no special GWModel case), and works correctly. * The special case in kickOutRewritable for [W] fsk ~ ty, turns out not to be needed. (We emit a [D] fsk ~ ty which will do the job. I improved comments and documentation, esp in TcSMonad. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11408#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler