#15799: GHC panic (and warnings) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #12045 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Urgh. I know what's going on. It has nothing to do with #12045, but perhaps changes there affected code paths in such a way that this surfaced. The problem is that we have a constraint `b <= a`. Note that `(<=) :: ty1 -> ty2 -> Type` -- that is, `b <= a` is ill-kinded as a constraint. So GHC processes it and dutifully wraps it up by casting by a coercion hole; we get `(b <= a) |> co`, where `co :: Type ~ Constraint`. Then, we zonk (with `zonkTcType`). The zonker uses `mapType` with `tcm_smart` set to `True`. This means that casts are reconstructed with `mkCastTy`, which, of course, does a reflexivity check. And, lo, the reflexivity check decides that `Type` and `Constraint` are equal and thus drops the cast. Later, we try to treat `b <= a` as an predicate, and GHC falls over when `isPredTy (b <= a)` returns `False`. What to do? My first instinct was to add a new setting to `tcm_smart`: sometimes we want to use raw constructors (in `nakedSubstMapper`), sometimes we want core smart constructors (in `zonkTcTypeToType`), and now sometimes we want type-checker smart constructors (newly discovered for `zonkTcType`). We'll also need a `mkTcCastTy` which is like `mkCastTy` but calls `isTcReflexiveCo` instead of `isReflexiveCo`, so that `Type` can be known to be distinct from `Constraint`. (Actually, `mkTcCastTy` can probably skip some of the steps in `mkCastTy`, as we know we'll get another crack at establishing invariants, etc., in `zonkTcTypeToType`.) This is a mess! So my second instinct was to post here for help. Help? Does anyone see a way out? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15799#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler