[GHC] #13943: Compiler infinite loop with GHC-8.2

[GHC]

#13943: Compiler infinite loop with GHC-8.2
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        Reporter:  vagarenko         |                Owner:  dfeuer
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.3
       Component:  Compiler          |              Version:  8.2.1-rc3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12791            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"a8fde1831f4b99885b8ed444f9cd7dffd9252150/ghc" a8fde18/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="a8fde1831f4b99885b8ed444f9cd7dffd9252150"
 Fix bug in the short-cut solver

 Trac #13943 showed that the relatively-new short-cut solver
 for class constraints (aka -fsolve-constant-dicts) was wrong.
 In particular, see "Type families" under Note [Shortcut solving]
 in TcInteract.

 The short-cut solver recursively solves sub-goals, but it doesn't
 flatten type-family applications, and as a result it erroneously
 thought that C (F a) cannot possibly match (C 0), which is
 simply untrue.  That led to an inifinte loop in the short-cut
 solver.

 The significant change is the one line

 +                 , all isTyFamFree preds  -- See "Type families" in
 +                                          -- Note [Shortcut solving]

 but, as ever, I do some other refactoring.  (E.g. I changed the
 name of the function to shortCutSolver rather than the more
 generic trySolveFromInstance.)

 I also made the short-cut solver respect the solver-depth limit,
 so that if this happens again it won't just produce an infinite
 loop.

 A bit of other refactoring, notably moving isTyFamFree
 from TcValidity to TcType
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13943#comment:12>
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