[GHC] #1012: ghc panic with mutually recursive modules and template haskell

[GHC]

#1012: ghc panic with mutually recursive modules and template haskell
-------------------------------------+-------------------------------------
        Reporter:  guest             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Template Haskell  |              Version:  6.8.2
      Resolution:                    |             Keywords:  hs-boot
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  TH_import_loop
      Blocked By:                    |             Blocking:
 Related Tickets:  #9032             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 This bug still exists in GHC 8.0, although it can be a bit tricky to
 trigger if the topsort picks an ordering of modules which masks the error.
 I found this especially surprising because in the parallel upsweep case we
 seem to try to block, c.f. this comment in GhcMake

 {{{
 2. A module that depends on a module in an external loop can't proceed
    until the entire loop is re-typechecked.
 }}}

 and this code:

 {{{
     -- If this module depends on a module within a loop then it must wait
 for
     -- that loop to get re-typechecked, i.e. it must wait on the module
 that
     -- finishes that loop. These extra dependencies are this module's
     -- "external" loop dependencies, because this module is outside of the
     -- loop(s) in question.
     let ext_loop_deps = Set.fromList
             [ head loop | loop <- comp_graph_loops
                         , any (`Set.member` textual_deps) loop
                         , this_build_mod `notElem` loop ]
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1012#comment:42>
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