Fix #1820: condition of whether generates outer

Previously, we don't generate `outer` for the annoymous class `Inner3`,
because it doesn't reference the symbol `B`.

This is incorrect, as `Inner3` extends `A`, which requires an outer.

trait A {
  val a = "a"
  trait Inner {
    def f = println(a)
    def h = 3
  }
}

trait B extends A {
  trait Inner2 extends Inner
  class Inner3 extends Inner2
}

Author: liufengyun

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

@@ -79,7 +79,8 @@ class ExplicitOuter extends MiniPhaseTransform with InfoTransformer { thisTransf
     val isTrait = cls.is(Trait)
     if (needsOuterIfReferenced(cls) &&
         !needsOuterAlways(cls) &&
-        impl.existsSubTree(referencesOuter(cls, _)))
+        (cls.mixins.exists(needsOuterIfReferenced) ||
+          impl.existsSubTree(referencesOuter(cls, _))))
       ensureOuterAccessors(cls)
 
     val hasOuterFlag = hasOuter(cls)

tests/run/i1820b.check

@@ -0,0 +1 @@
+a

tests/run/i1820b.scala

@@ -0,0 +1,19 @@
+trait A {
+  val a = "a"
+  trait Inner {
+    def f = println(a)
+    def h = 3
+  }
+}
+
+trait B extends A {
+  trait Inner2 extends Inner
+  def g = new Inner2 {}
+}
+
+object O {
+  def main(args: Array[String]): Unit = {
+    val b = new B {}
+    b.g.f
+  }
+}