Merge pull request #4164 from retronym/ticket/9003

SI-9003 Eagerly capture more potentially mutable binders

2 files changed, 9 insertions, 1 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,
diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
index 3abec521df..3fd9ce76f8 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
@@ -192,13 +192,14 @@ trait MatchTreeMaking extends MatchCodeGen with Debugging {
     case class ExtractorTreeMaker(extractor: Tree, extraCond: Option[Tree], nextBinder: Symbol)(
           val subPatBinders: List[Symbol],
           val subPatRefs: List[Tree],
+          val potentiallyMutableBinders: Set[Symbol],
           extractorReturnsBoolean: Boolean,
           val checkedLength: Option[Int],
           val prevBinder: Symbol,
           val ignoredSubPatBinders: Set[Symbol]
           ) extends FunTreeMaker with PreserveSubPatBinders {
 
-      def extraStoredBinders: Set[Symbol] = Set()
+      def extraStoredBinders: Set[Symbol] = potentiallyMutableBinders
 
       debug.patmat(s"""
         |ExtractorTreeMaker($extractor, $extraCond, $nextBinder) {