fix #1820: make sure outer of traits implemented

Author: liufengyun

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

@@ -81,14 +81,19 @@ class ExplicitOuter extends MiniPhaseTransform with InfoTransformer { thisTransf
         !needsOuterAlways(cls) &&
         impl.existsSubTree(referencesOuter(cls, _)))
       ensureOuterAccessors(cls)
-    if (hasOuter(cls)) {
+
+    val hasOuterFlag = hasOuter(cls)
+    if (hasOuterFlag || cls.mixins.exists(needsOuterIfReferenced)) {
       val newDefs = new mutable.ListBuffer[Tree]
-      if (isTrait)
-        newDefs += DefDef(outerAccessor(cls).asTerm, EmptyTree)
-      else {
-        val outerParamAcc = outerParamAccessor(cls)
-        newDefs += ValDef(outerParamAcc, EmptyTree)
-        newDefs += DefDef(outerAccessor(cls).asTerm, ref(outerParamAcc))
+
+      if (hasOuterFlag) {
+        if (isTrait)
+          newDefs += DefDef(outerAccessor(cls).asTerm, EmptyTree)
+        else {
+          val outerParamAcc = outerParamAccessor(cls)
+          newDefs += ValDef(outerParamAcc, EmptyTree)
+          newDefs += DefDef(outerAccessor(cls).asTerm, ref(outerParamAcc))
+        }
       }
 
       for (parentTrait <- cls.mixins) {

tests/run/i1820.check

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

tests/run/i1820.scala

@@ -0,0 +1,17 @@
+class A {
+  val a = "a"
+  trait Inner {
+    def f = println(a)
+    def h = 3
+  }
+}
+
+class Inner extends O.a.Inner
+
+object O {
+  val a = new A
+
+  def main(args: Array[String]): Unit = {
+    (new Inner).f
+  }
+}