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If you compile this program:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE GADTs #-}

import GHC.Exts (Constraint)

data Dict a where
  Dict :: a => Dict a

foo :: Dict (Int ~ Int) => Int
foo = undefined

In GHC 8.0.1, 8.0.2, 8.2.1, or HEAD, it will give this error message:

GHCi, version 8.3.20170509: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug2.hs, interpreted )

Bug2.hs:9:8: error:
    • Expected a constraint, but ‘Dict Int ~ Int’ has kind ‘*’
    • In the type signature: foo :: Dict (Int ~ Int) => Int
  |
9 | foo :: Dict (Int ~ Int) => Int
  |        ^^^^^^^^^^^^^^^^

But Dict Int ~ Int is not parenthesized correctly!

This is a regression, as GHC 7.10.3 and earlier parenthesize it correctly:

GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( Bug2.hs, interpreted )

Bug2.hs:9:8:
    Expected a constraint, but ‘Dict (Int ~ Int)’ has kind ‘*’
    In the type signature for ‘foo’: foo :: Dict (Int ~ Int) => Int

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture