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

@@ -65,5 +65,6 @@ class AugmentScala2Traits extends MiniPhase with IdentityDenotTransformer { this
         sym.ensureNotPrivate.installAfter(thisPhase)
     }
     mixin.setFlag(Scala2xPartiallyAugmented)
+    mixin.transformAfter(thisPhase, d => { d.setFlag(Scala2xPartiallyAugmented); d })
   }
 }

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