Fix #4559: Don't force initializers of lazy mixin fields

Author: odersky

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

@@ -214,7 +214,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
           EmptyTree
       }
 
-      for (getter <- mixin.info.decls.toList if getter.isGetter && !was(getter, Deferred)) yield {
+      for (getter <- mixin.info.decls.toList if getter.isGetter && !was(getter, Deferred | Lazy)) yield {
         val isScala2x = mixin.is(Scala2x)
         def default = Underscore(getter.info.resultType)
         def initial = transformFollowing(superRef(initializer(getter)).appliedToNone)

tests/run/i4559.scala

@@ -0,0 +1,9 @@
+trait A {
+  lazy val x = { println("super[A].x()"); 123 }
+}
+
+class B extends A {
+  override lazy val x = 456
+}
+
+object Test { def main(args: Array[String]): Unit = { new B() } }