AugmentScala2Traits: Handle traits that erase to a Scala 2 trait

There is only one of those so far: NonEmptyTuple which erases to
Product. Without this commit, the mixin forwarders to Product in the *:
class in Tuple.scala were only generated if we happened to have seen
augmented Product in a previous compilation unit in the same run.

Author: smarter

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -36,6 +36,16 @@ object TypeErasure {
   private def erasureDependsOnArgs(sym: Symbol)(implicit ctx: Context) =
     sym == defn.ArrayClass || sym == defn.PairClass
 
+  def normalizeClass(cls: ClassSymbol)(implicit ctx: Context): ClassSymbol = {
+    if (cls.owner == defn.ScalaPackageClass) {
+      if (defn.specialErasure.contains(cls))
+        return defn.specialErasure(cls)
+      if (cls == defn.UnitClass)
+        return defn.BoxedUnitClass
+    }
+    cls
+  }
+
   /** A predicate that tests whether a type is a legal erased type. Only asInstanceOf and
    *  isInstanceOf may have types that do not satisfy the predicate.
    *  ErasedValueType is considered an erased type because it is valid after Erasure (it is
@@ -530,16 +540,6 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       this(tp)
   }
 
-  private def normalizeClass(cls: ClassSymbol)(implicit ctx: Context): ClassSymbol = {
-    if (cls.owner == defn.ScalaPackageClass) {
-      if (defn.specialErasure.contains(cls))
-        return defn.specialErasure(cls)
-      if (cls == defn.UnitClass)
-        return defn.BoxedUnitClass
-    }
-    cls
-  }
-
   /** The name of the type as it is used in `Signature`s.
    *  Need to ensure correspondence with erasure!
    */

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

@@ -39,8 +39,11 @@ class AugmentScala2Traits extends MiniPhase with IdentityDenotTransformer { this
 
   override def transformTemplate(impl: Template)(implicit ctx: Context): Template = {
     val cls = impl.symbol.owner.asClass
-    for (mixin <- cls.mixins if mixin.is(Scala2x) && !mixin.is(Scala2xPartiallyAugmented))
-      augmentScala2Trait(mixin)
+    for (mixin <- cls.mixins) {
+      val erasedMixin = TypeErasure.normalizeClass(mixin)
+      if (erasedMixin.is(Scala2x) && !erasedMixin.is(Scala2xPartiallyAugmented))
+        augmentScala2Trait(erasedMixin)
+    }
     impl
   }