#13988: GADT constructor with kind equality constraint quantifies unused existential type variables -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Reproducible with GHC 8.0.1., 8.0.2, 8.2.1, or HEAD (but I'll use GHC 8.2.1 to show the results of `:type +v`): {{{ $ /opt/ghc/8.2.1/bin/ghci GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :set -XTypeInType -XGADTs -fprint-explicit-foralls λ> import Data.Kind λ> data Foo (a :: k) where MkFoo :: (k ~ Type) => Foo (a :: k) λ> :type +v MkFoo MkFoo :: forall k1 (a :: k1) k2 (k3 :: k2). k1 ~ * => Foo a }}} Note that `MkFoo` quantifies two extra existential type variables, `k2` and `k3` which are completely unused! Note that this does not occur if the `MkFoo` constructor uses an explicit `forall`: {{{ λ> :set -XScopedTypeVariables λ> data Foo (a :: k) where MkFoo :: forall k (a :: k). (k ~ Type) => Foo (a :: k) λ> :type +v MkFoo MkFoo :: forall k (a :: k). k ~ * => Foo a }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13988 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler