[GHC] #14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE instance

[GHC]

#14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE
instance
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        Reporter:  dredozubov        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.3
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Incorrect result  |  (amd64)
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"30058b0e45e920319916be999de9c4d77da136e7/ghc"
 30058b0e/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="30058b0e45e920319916be999de9c4d77da136e7"
 Fix another dark corner in the shortcut solver

 The shortcut solver for type classes (Trac #12791) was eagerly
 solving a constaint from an OVERLAPPABLE instance. It happened
 to be the only one in scope, so it was unique, but since it's
 specfically flagged as overlappable it's really a bad idea to
 solve using it, rather than using the Given dictionary.

 This led to Trac #14434, a nasty and hard to identify bug.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14434#comment:5>
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