Non-deterministic warnings from virtpatmat

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Compiling the following three tests generate different output on each run:

test/files/run/virtpatmat_alts.scala
test/files/run/virtpatmat_nested_lists.scala
test/files/run/virtpatmat_opt_sharing.scala

Currently, partest doesn't see these warnings because of #7003, so the tests pass.

An example:

szabi@kama3 ~/dev/git/scala > qscalac test/files/run/virtpatmat_alts.scala
test/files/run/virtpatmat_alts.scala:2: warning: match may not be exhaustive.
It would fail on the following inputs: (false, true), (true, false)
  (true, true) match {
  ^
test/files/run/virtpatmat_alts.scala:6: warning: match may not be exhaustive.
It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7)), _), List((x: Int forSome x not in (1, 4, 5, 6, 7)), _), List(1, _), List(2, _), List(4, _), List(5, _), List(6, _), List(7, _)
  List(5) match {
      ^
two warnings found

szabi@kama3 ~/dev/git/scala > qscalac test/files/run/virtpatmat_alts.scala
test/files/run/virtpatmat_alts.scala:2: warning: match may not be exhaustive.
It would fail on the following inputs: (false, true), (true, false)
  (true, true) match {
  ^
test/files/run/virtpatmat_alts.scala:6: warning: match may not be exhaustive.
It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List((x: Int forSome x not in (1, 2, 4, 5, 6, 7)), _), List((x: Int forSome x not in (1, 2, 4, 5, 7)), _), List(1, _), List(2, _), List(4, _), List(5, _), List(6, _), List(7, _)
  List(5) match {
      ^
two warnings found

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Imported From: https://issues.scala-lang.org/browse/SI-7020?orig=1
Reporter: @khernyo
Affected Versions: 2.10.0

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@retronym said:
This seems to return us to the realm of determinism: https://github.com/retronym/scala/compare/ticket/7020

I still need to add the other tests and also figure out which of those sets/maps actually needs to be linked rather than the heavy handed approach I took.

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@retronym said:
scala/scala#2826

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@retronym said:
Raising to critical, non determinism is the enemy of regression testing, and this one came up in @gbasler's recent pattern matcher rework.

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@adriaanm said:
Awesome -- thanks!

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@adriaanm said:
It didn't fix the tseitin refactor. Looking into that now.

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@retronym said:
Reopening: The non-determinism monster remains at large: https://github.com/scala/scala/pull/3043/files

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@retronym said:
I could reproduce the error:

# Generate some clones of the test
% for i in {1..100}; do cp test/disabled/neg/t7020.scala test/disabled/neg/t7020-$i.scala; cp test/disabled/neg/t7020.flags test/disabled/neg/t7020-$i.flags; cat test/disabled/neg/t7020.check | sed "s/t7020/t7020-$i/" > test/disabled/neg/t7020-$i.check; done

/code/scala test/partest --terse --srcpath disabled test/disabled/neg/t7020*.scala
Selected 101 tests drawn from specified tests

# starting 101 tests in neg
........................................................................
.............................

101/101 passed (elapsed time: 00:00:34)
Test Run PASSED
ticket/7020-2 /code/scala test/partest --terse --srcpath disabled test/disabled/neg/t7020*.scala
Selected 101 tests drawn from specified tests

# starting 101 tests in neg
........................................................................
.............................

101/101 passed (elapsed time: 00:00:34)
Test Run PASSED
ticket/7020-2 /code/scala test/partest --terse --srcpath disabled test/disabled/neg/t7020*.scala
Selected 101 tests drawn from specified tests

# starting 101 tests in neg
........................................................................
.............................

101/101 passed (elapsed time: 00:00:34)
Test Run PASSED
ticket/7020-2 /code/scala test/partest --terse --srcpath disabled test/disabled/neg/t7020*.scala
Selected 101 tests drawn from specified tests

# starting 101 tests in neg
........................................................................
..
!!   1 - neg/t7020-76.scala                        [output differs]
..........................
# 100/101 passed, 1 failed in neg

# Failed test paths (this command will update checkfiles)
test/partest --update-check \
  /Users/jason/code/scala/test/disabled/neg/t7020-76.scala

100/101 passed, 1 failed (elapsed time: 00:00:32)
Test Run FAILED

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@retronym said:
This fixes the non-determinism: https://github.com/retronym/scala/compare/ticket/7020-3

I'm still trying to figure out which parts of the patch actually fix the problem.

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@retronym said:
scala/scala#3068