GADT constructor with kind equality constraint quantifies unused existential type variables
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 aNote 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 aTrac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
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