#15918: GHC panic from QuantifiedConstraints(?) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 Resolution: | Keywords: | QuantifiedConstraints 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 simonpj): This is another symptom of the bug identified in #15799. In the example of this ticket we have {{{ Elaborated types Rev :: forall k. (k->*) -> Constraint Build :: forall k. ((k->*) -> Constraint) -> k -> * rev :: forall k (f :: k->*) (a :: k). Rev @k f => f a instance forall k (cls :: (k->*) -> Constraint). (forall (xx :: k->*). cls xx => Rev @k xx) => Rev @k (Build @k cls) From the RHS of uu: [W] Rev @k (Build ([] |> co)) [W] co :: (* -> *) ~ ((k->*) -> Constraint) }}} Decompose co, we get {{{ [W] Rev @k (Build ([] |> (co1 -> co2))) [W] co1 :: * ~ (k->*) [W] co2 :: * ~ Constraint }}} Now the first constraint matches the instance, so we get a substitution: {{{ cls :-> ([] |> (co1 -> co2) }}} Now * we apply `substTheta` (this is in `Inst.instDFunType`) to `(cls xx)` * `substTheta` calls `mkAppTy ([] |> (co1 -> co2)) xx` * `mkAppTy` uses `decomposePiCos` (which works) and then calls `mkCastTy` (twice) * Alas, `mkCastTy` considers that `Constraint ~ *`, and we lose `co2` And that's how the assert fails. The underlying cause is the same: `mkCastTy` is using `isReflexiveCo`, and we really want `mkTcCastTy` here. But we don't really want to have a `Tc` variant of `substTy`! Sigh. So much pain from the varying treatment of `Constraint` and `Type`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15918#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler