[GHC] #12042: Infinite loop with type synonyms and hs-boot

[GHC]

#12042: Infinite loop with type synonyms and hs-boot
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:  hs-boot
      Resolution:                    |  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2656
       Wiki Page:                    |
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Comment (by Edward Z. Yang <ezyang@…>):

 In [changeset:"31398fbc6d9ee0bd95de64b08becc38faf188972/ghc"
 31398fbc/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="31398fbc6d9ee0bd95de64b08becc38faf188972"
 Test for type synonym loops on TyCon.

 Summary:
 Previously, we tested for type synonym loops by doing
 a syntactic test on the literal type synonym declarations.
 However, in some cases, loops could go through hs-boot
 files, leading to an infinite loop (#12042); a similar
 situation can occur when signature merging.

 This commit replaces the syntactic test with a test on
 TyCon, simply by walking down all type synonyms until
 we bottom out, or find we've looped back.  It's a lot
 simpler.

 Signed-off-by: Edward Z. Yang <ezyang at cs.stanford.edu>

 Test Plan: validate

 Reviewers: simonpj, austin, bgamari

 Subscribers: goldfire, thomie

 Differential Revision: https://phabricator.haskell.org/D2656

 GHC Trac Issues: #12042
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12042#comment:3>
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