Flags can't be used as non-monotonic signals between phases

since denotations are phase-versioned and later denotations
might exist already when the flag of an earlier denotation is
set. To avoid this, set flags in all following denotations,
not just the current one.

Author: odersky

compiler/src/dotty/tools/dotc/transform/AugmentScala2Traits.scala

@@ -64,6 +64,6 @@ class AugmentScala2Traits extends MiniPhase with IdentityDenotTransformer { this
         || sym.isSuperAccessor) // scala2 superaccessors are pickled as private, but are compiled as public expanded
         sym.ensureNotPrivate.installAfter(thisPhase)
     }
-    mixin.setFlag(Scala2xPartiallyAugmented)
+    mixin.transformAfter(thisPhase, d => { d.setFlag(Scala2xPartiallyAugmented); d })
   }
 }

compiler/src/dotty/tools/dotc/transform/LinkScala2Impls.scala

@@ -60,7 +60,7 @@ class LinkScala2Impls extends MiniPhase with IdentityDenotTransformer { thisPhas
       if (needsMixinForwarder(sym) || sym.isConstructor || sym.isGetter && sym.is(Lazy) || sym.is(Method, butNot = Deferred))
         newImpl(sym.asTerm).enteredAfter(thisPhase)
     // The trait is now fully augmented so the flag isn't needed anymore.
-    mixin.resetFlag(Scala2xPartiallyAugmented)
+    mixin.transformAfter(thisPhase, d => { d.resetFlag(Scala2xPartiallyAugmented); d })
   }
 
   override def prepareForTemplate(impl: Template)(implicit ctx: Context): Context = {

tests/run/i8101/Foo.scala

@@ -0,0 +1,3 @@
+trait Foo {
+  def f: String = ???
+}

tests/run/i8101/JavaFoo.java

@@ -0,0 +1,3 @@
+public abstract class JavaFoo {
+    public abstract int read();
+}

tests/run/i8101/Test.scala

@@ -0,0 +1,6 @@
+class Bar extends JavaFoo with Foo {
+  def read(): Int = ???
+}
+
+@main def Test =
+  val stdout = new Bar