SI-9003 Eagerly capture more potentially mutable binders

This is a re-run of SI-5158 in two different contexts.

As reported, the result of `unapply[Seq]` under name based pattern
matching might not guarantee stability of results of calls to
`_1`, `apply(i)`, etc, so we call these eagerly, and call them
once only.

I also found a case in regular pattern matching that we hadn't
accounted for: extracting elements of sequences (either from
a case class or from an `unapplySeq`) may also be unstable.

This commit changes `ExtractorTreeMaker` to force storage
of such binders, even under `-optimize`. This parallels the change
to `ProductExtractorTreeMaker` in 8ebe8e3e8.

I have added a special case for traditional `unapply` methods
returning `Option`. This avoids a change for:

```
% cat test/files/run/t9003b.scala
object Single {
  def unapply(a: Any) = Some("")
}

object Test {
  def main(args: Array[String]): Unit = {
    "" match {
      case Single(x) =>
        (x, x)
    }
  }
}
% qscalac -optimize -Xprint:patmat test/files/run/t9003b.scala 2>&1 | grep --context=5 get
      case <synthetic> val x1: Any = "";
      case5(){
        <synthetic> val o7: Some[String] = Single.unapply(x1);
        if (o7.isEmpty.unary_!)
          matchEnd4({
            scala.Tuple2.apply[String, String](o7.get, o7.get);
            ()
          })
        else
          case6()
      };
% scalac-hash v2.11.4 -optimize -Xprint:patmat test/files/run/t9003b.scala 2>&1 | grep --context=5 get
      case <synthetic> val x1: Any = "";
      case5(){
        <synthetic> val o7: Some[String] = Single.unapply(x1);
        if (o7.isEmpty.unary_!)
          matchEnd4({
            scala.Tuple2.apply[String, String](o7.get, o7.get);
            ()
          })
        else
          case6()
      };
```

1 files changed, 7 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
index d862805a07..22661d6ccf 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
@@ -544,10 +544,17 @@ trait MatchTranslation {
         // wrong when isSeq, and resultInMonad should always be correct since it comes
         // directly from the extractor's result type
         val binder         = freshSym(pos, pureType(resultInMonad))
+        val potentiallyMutableBinders: Set[Symbol] =
+          if (extractorApply.tpe.typeSymbol.isNonBottomSubClass(OptionClass) && !aligner.isSeq)
+            Set.empty
+          else
+            // Ensures we capture unstable bound variables eagerly. These can arise under name based patmat or by indexing into mutable Seqs. See run t9003.scala
+            subPatBinders.toSet
 
         ExtractorTreeMaker(extractorApply, lengthGuard(binder), binder)(
           subPatBinders,
           subPatRefs(binder),
+          potentiallyMutableBinders,
           aligner.isBool,
           checkedLength,
           patBinderOrCasted,