Fix #2883: Avoid capturing fields accessed in constructor

In LambdaLift, fields used only in the constructor of a class should not be marked
free in the enclosing class.

Author: odersky

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

@@ -202,6 +202,7 @@ class LambdaLift extends MiniPhase with IdentityDenotTransformer { thisTransform
         ctx.debuglog(i"mark free: ${sym.showLocated} with owner ${sym.maybeOwner} marked free in $enclosure")
         val intermediate =
           if (enclosure.is(PackageClass)) enclosure
+          else if (enclosure.isConstructor) markFree(sym, enclosure.owner.enclosure)
           else markFree(sym, enclosure.enclosure)
         narrowLiftedOwner(enclosure, intermediate orElse sym.enclosingClass)
         if (!intermediate.isRealClass || enclosure.isConstructor) {

tests/run/i2883.check

@@ -0,0 +1 @@
+

tests/run/i2883.scala

@@ -0,0 +1,16 @@
+class Wrapper(val value: Int)
+
+abstract class Foo(val x: Int)
+
+class Test {
+  def foo(wrapper: Wrapper): Unit = {
+    new Foo(wrapper.value) {}
+  }
+}
+object Test extends App {
+  def foo(wrapper: Wrapper): Foo =
+    new Foo(wrapper.value) {}
+  def printFields(obj: Any) =
+    println(obj.getClass.getDeclaredFields.map(_.toString).sorted.deep.mkString("\n"))
+  printFields(foo(new Wrapper(1)))
+}