[GHC] #14402: Compiler hangs - UdecidableInstances

[GHC]

#14402: Compiler hangs - UdecidableInstances
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           Reporter:  lehins         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Normally I wouldn't consider GHC going into infinite loop when
 `UndecidableInstances` is enabled as a bug, because compiler still
 terminates unless maybe something like `-freduction-depth=0` is supplied.
 That is not the case with this few lines of Haskell, as they cause GHC to
 hang for good and even made it use up all of my 32Gb of RAM, while I was
 looking the other way. Seems a bit unusual for GHC, so here is a record of
 it as a bug report. Compiling with `-v3` stops here:

 {{{
 [1 of 1] Compiling Nested           ( src/Nested.hs, .stack-
 work/dist/x86_64-linux/Cabal-2.0.0.2/build/Nested.o )
 *** Parser [Nested]:
 !!! Parser [Nested]: finished in 1.03 milliseconds, allocated 0.566
 megabytes
 *** Renamer/typechecker [Nested]:
 }}}

 Compilation with GHC 8.0.2 goes through just fine. Although, I did
 encounter it hanging as part of a much bigger picture because of the same
 few lines of code. Coming up with a small reproducible example is much
 harder for that case as it hangs while compiling the test suite, instead
 of the offending module.

 In order to replicate the problem, the included code has to be compiled as
 a library rather than as an executable. Using `stack build --install-ghc`
 should do it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14402>
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