Merge pull request #4636 from dotty-staging/fix-#4559

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

Author: liufengyun

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

@@ -242,7 +242,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
           // transformFollowing call is needed to make memoize & lazy vals run
           transformFollowing(DefDef(implementation(getter.asTerm), rhs))
         }
-        else if (isScala2x || was(getter, ParamAccessor)) EmptyTree
+        else if (isScala2x || was(getter, ParamAccessor | Lazy)) EmptyTree
         else initial
       }
     }

tests/run/i4559.check

@@ -0,0 +1,2 @@
+super[A] init
+x()

tests/run/i4559.scala

@@ -0,0 +1,10 @@
+trait A {
+  println(s"super[A] init")
+  lazy val x = { println("super[A].x()"); 123 }
+}
+
+class B extends A {
+  override lazy val x = { println("x()"); 456 }
+}
+
+object Test { def main(args: Array[String]): Unit = { new B().x } }