Make module classes in traits non-private in Mixin.

They will be instantiated in classes that mix in their owner trait,
so they must be visible there.

Author: sjrd

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

@@ -134,6 +134,10 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
         else sym.copySymDenotation(initFlags = sym.flags &~ ParamAccessor | Deferred)
       sym1.ensureNotPrivate
     }
+    else if sym.isAllOf(ModuleClass | Private) && sym.owner.is(Trait) then
+      // modules in trait will be instantiated in the classes mixing in the trait; they must be made non-private
+      // do not use ensureNotPrivate because the `name` must not be expanded in this case
+      sym.copySymDenotation(initFlags = sym.flags &~ Private)
     else if (sym.isConstructor && sym.owner.is(Trait))
       sym.copySymDenotation(
         name = nme.TRAIT_CONSTRUCTOR,