[GHC] #7788: Recursive type family causes <<loop>>

[GHC]

#7788: Recursive type family causes <<loop>>
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        Reporter:  shachaf           |                   Owner:  simonpj
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.10.1
       Component:  Compiler (Type    |                 Version:  7.6.2
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  Incorrect result  |               Test Case:
  at runtime                         |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
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Comment (by int-e):

 Replying to [comment:6 goldfire]:
 > But, with 7.10, the following doesn't:
 >
 > {{{
 > type family F a where
 >   F a = F a
 >
 > id2 :: F a -> F a
 > id2 x = x
 > }}}
 >
 > This is slightly disturbing because `id2` is merely a specialization of
 `id`.

 I think this is wrong: `id2` does not have a type at all, and therefore
 does not specialize `id`.

 The same is true for the original example. If one looks at the code
 generated by ghc-7.8.3, it tries to produce a witness for {{{F (Fix Id)}}}
 being a type, and `foo` checks that witness (which fails, producing the
 observed {{{<<loop>>}}}) before producing `undefined`:

 {{{
 -- core, with some non-essential parts omitted

 cobox_rVz :: Main.F (Main.Fix Main.Id) ~ GHC.Prim.Any
 cobox_rVz = cobox_rVz

 foo :: Main.Proxy (Main.F (Main.Fix Main.Id)) -> ()
 foo = case cobox_rVz of GHC.Types.Eq# _ -> undefined
 }}}

 That said it's unfortunate that GHC 7.10 loops rather than erroring out
 for these examples.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7788#comment:10>
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