#15361: Error message displays redundant equality constraint -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.4.3 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #14394 | Differential Rev(s): Phab:D5002 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I was puzzled about why redundant givens are sometimes reported and sometimes not. Here's the deal * For each implication GHC figures out, during constraint solving, whether the implication binds any equalities. This is used to guide equality float-out, and is recorded in the implication in the `ic_no_eqs` field. * The "whether binds any equalities" test is performed on the ''canonicalised, inert givens'', in `TcSMonad.getNoGivenEqs`. See `Note [When does an implication have given equalities?]`. * If an implication binds `* ~ *` only (call it situation (A)), we'll discard that Given, and mark the implication as `ic_no_eqs = True`. * But if an implication binds `* ~ *` ''and'' `a ~ b`, situation (B), the `ic_no_eqs` field will be set `False`. * When reporting an equality error, we report givens only from implications with `ic_no_eqs` = `False`. That makes sense. But it'll discard situation (A), and keep situation (B). * For the implications we keep, we report all original, user-written givens. Hence the reported `* ~ *`. So that explain the behavior you see. For example for `hoo` in comment:2, the `Int :~: Int` binds no equalities because when `Int ~ Int` is solved we get nothing. So it is not reported. I suppose, therefore, that the solution you suggest is no unreasonable, ''provided'' you write Note to explain. In particular * `mkMinimalBySCs` happens to eliminate reflexive equalities, as well as superclasses. (Why? See Note [Remove redundant provided dicts]` in `TcPatSyn`. * Explain how a given `* ~ *` can arise. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15361#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler