#15621: Error message involving type families points to wrong location -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.4.3 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): We have `Refl :: forall a. Refl a a` So from the defn of `a` we instantiate `Refl` with `a := alpha`, and get {{{ [W] c1 :: alpha ~ F Int -- Line 12 [W] c2 :: alpha ~ F Int -- Line 12 }}} Each constraint carries its birth location, which I show here. Same for `b`, instantiating `Refl` with `a := beta`: {{{ [W] c3 :: beta ~ F Int -- Line 15 [W] c4 :: beta ~ F Bool -- Line 15 }}} Then depending on the order in which we try to solve, we might do {{{ alpha := beta beta := F Bool -- solves c4 }}} Now, substituting for these unified varaiables, we get {{{ [W] c1 :: F Bool ~ F Int -- Line 12 [W] c2 :: F Bool ~ F Int -- Line 12 [W] C3 :: F Bool ~ F Int -- Line 15 }}} Since these are identical, we pick one; and alas, it is `c1`. So that's why we get a bogus message. Richard, I'd be interested in your views. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15621#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler