SI-7269 Rework MapLike#retains to account for desugaring change

`MapLike#retains` contains a for-comprehension that relied on the strict
`filter` by its generator. You can't, in general, iterate a mutable map
and remove items in the same pass.

Here's the history of the desugaring of:

    def retain[A, B](thiz: mutable.Map[A, B])(p: (A, B) => Boolean): thiz.type = {
      thiz.foreach {
        case (k, v) =>
          if (p(k, v)) thiz -= k
      }

Before regression (c82ecabad6~1):

    thiz.filter(((check$ifrefutable$1) => check$ifrefutable$1: @scala.unchecked match {
       case scala.Tuple2((k @ _), (v @ _)) => true
       case _ => false
    })).withFilter(((x$1) => x$1: @scala.unchecked match {
       case scala.Tuple2((k @ _), (v @ _)) => p(k, v).unary_$bang
    })).foreach(((x$2) => x$2: @scala.unchecked match {
       case scala.Tuple2((k @ _), (v @ _)) => thiz.$minus$eq(k)
    }));

After regression (c82ecabad6, which incorrectly assumed in the parser that
no filter is required for isInstanceOf[Tuple2])

    thiz.withFilter(((x$1) => x$1: @scala.unchecked match {
       case scala.Tuple2((k @ _), (v @ _)) => p(k, v).unary_$bang
    })).foreach(((x$2) => x$2: @scala.unchecked match {
      case scala.Tuple2((k @ _), (v @ _)) => thiz.$minus$eq(k)
    }));

After the reversion of c82ecabad6, v2.10.2
This is also after 365bb2b4e, which uses `withFilter` rather than `filter`.

    thiz.withFilter(((check$q$1) => check$ifrefutable$1: @scala.unchecked match {
       case scala.Tuple2((k @ _), (v @ _)) => true
       case _ => false
    })).withFilter(((x$1) => x$1: @scala.unchecked match {
       case scala.Tuple2((k @ _), (v @ _)) => p(k, v).unary_$bang
    })).foreach(((x$2) => x$2: @scala.unchecked match {
      case scala.Tuple2((k @ _), (v @ _)) => thiz.$minus$eq(k)
    }));

This commit does the same as `SetLike#retains`, and converts the map to
an immutable list before the rest of the operation.

1 files changed, 32 insertions, 0 deletions

diff --git a/test/files/run/t7269.scala b/test/files/run/t7269.scala
new file mode 100644
index 0000000000..d22e57dfee
--- /dev/null
+++ b/test/files/run/t7269.scala
@@ -0,0 +1,32 @@
+import scala.collection.JavaConversions._
+import scala.collection.mutable
+
+object Test extends App {
+
+  def testMap(): Unit = {
+    val mapJ = new java.util.HashMap[Int, String]
+    val mapS: mutable.Map[Int, String] = mapJ
+
+    (10 to 20).foreach(i => mapS += ((i, i.toString)))
+    assert(11 == mapS.size)
+
+    // ConcurrentModificationException thrown in the following line
+    mapS.retain((i, str) => i % 2 == 0)
+    assert(6 == mapS.size)
+  }
+
+  def testSet(): Unit = {
+    val mapJ = new java.util.HashSet[Int]
+    val mapS: mutable.Set[Int] = mapJ
+
+    (10 to 20).foreach(i => mapS += i)
+    assert(11 == mapS.size)
+
+    // ConcurrentModificationException thrown in the following line
+    mapS.retain((i) => i % 2 == 0)
+    assert(6 == mapS.size)
+  }
+
+  testSet()
+  testMap()
+}