Fix #4866: do not lift try with no catch block

Forward port of scala/scala#922

Author: abeln

compiler/src/dotty/tools/dotc/transform/LiftTry.scala

@@ -20,6 +20,12 @@ import util.Store
  *  is lifted to
  *
  *      { def liftedTree$n() = try body catch handler; liftedTree$n() }
+ *
+ * However, don't lift try's without catch expressions (try-finally).
+ * Lifting is needed only for try-catch expressions that are evaluated in a context
+ * where the stack might not be empty. `finally` does not attempt to continue evaluation
+ * after an exception, so the fact that values on the stack are 'lost' does not matter
+ * (copied from https://github.com/scala/scala/pull/922).
  */
 class LiftTry extends MiniPhase with IdentityDenotTransformer { thisPhase =>
   import ast.tpd._
@@ -58,7 +64,7 @@ class LiftTry extends MiniPhase with IdentityDenotTransformer { thisPhase =>
     liftingCtx(false)
 
   override def transformTry(tree: Try)(implicit ctx: Context): Tree =
-    if (needLift) {
+    if (needLift && tree.cases.nonEmpty) {
       ctx.debuglog(i"lifting tree at ${tree.pos}, current owner = ${ctx.owner}")
       val fn = ctx.newSymbol(
         ctx.owner, LiftedTreeName.fresh(), Synthetic | Method,

tests/run/t4866.check

@@ -0,0 +1,2 @@
+Foo #lifted: 0
+FooLifted #lifted: 1

tests/run/t4866.scala

@@ -0,0 +1,27 @@
+// Test that try-finally aren't lifted, but try-catch are.
+
+class Foo(param: => Object) {
+  val field = try param finally ()
+}
+
+class FooLifted(param: => Object) {
+  val field = try param catch { case e: Exception => () } finally () 
+}
+
+object Test extends App {
+  def numLifted(o: Object): Int = {
+    var lifted = 0
+    
+    for (f <- o.getClass.getDeclaredMethods) {
+      f.setAccessible(true)
+      if (f.getName.startsWith("lifted")) {
+        lifted += 1
+      }
+    }
+
+    lifted
+  }
+  
+  println("Foo #lifted: %d".format(numLifted(new Foo(new Object))))
+  println("FooLifted #lifted: %d".format(numLifted(new FooLifted(new Object))))
+}