[GHC] #10806: Type error and type level (<=) together cause GHC to hang

[GHC]

#10806: Type error and type level (<=) together cause GHC to hang
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        Reporter:  htebalaka         |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.2
  checker)                           |
      Resolution:                    |                Keywords:
Operating System:  MacOS X           |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Description changed by htebalaka:

Old description:

> The following incorrect type in the function wrongArity triggers an
> infinite loop at compile time, though only in the presence of
> triggersLoop. The issue is somehow related to the use of (<=) in
> constraints of the Q data constructor; if I remove either of the
> constraints or add an (a <= c) constraint it works as you would expect.
> {{{#!hs
>
> {-# LANGUAGE GADTs, ExplicitNamespaces, TypeOperators, DataKinds  #-}
>
> import GHC.TypeLits (Nat, type (<=))
>
> data Q a where
>     Q :: (a <= b, b <= c) => proxy a -> proxy b -> Q c
>
> wrongArity :: a -> a
> wrongArity _ a = a
>
> triggersLoop :: Q b -> Q b -> Bool
> triggersLoop (Q _ _) (Q _ _) = undefined
> }}}

New description:

 The following incorrect arity triggers an infinite loop at compile time.
 The issue is somehow related to the use of (<=) in constraints of the Q
 data constructor; if I remove either of the constraints or add an (a <= c)
 constraint it works as you would expect.
 {{{#!hs

 {-# LANGUAGE GADTs, ExplicitNamespaces, TypeOperators, DataKinds  #-}

 import GHC.TypeLits (Nat, type (<=))

 data Q a where
     Q :: (a <= b, b <= c) => proxy a -> proxy b -> Q c

 triggersLoop :: Q b -> Q b -> Bool
 triggersLoop (Q _ _) (Q _ _) = print 'x' 'y'
 }}}

 Incorrect function definitions, like {{{foo :: a -> a ; foo _ x = x}}}
 also result in an infinite loop.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10806#comment:2>
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