Future-proof the collections for Dotty

Just like what we did for concat in
scala#7696, we add a dummy parameter list
to avoid signature clashes between mixin forwarders, but this change
should be less controversial since it only affects a deprecated method.

Note that this isn't actually needed to compile with Dotty master yet
because we only emit mixin forwarders when required to, but we will soon
switch to emitting them all the time like scalac
does (scala/scala3#2563), therefore we need
to get the fix in first (and most importantly in time for 2.13).

Author: smarter

src/library/scala/collection/Map.scala

@@ -311,8 +311,12 @@ trait MapOps[K, +V, +CC[_, _] <: IterableOps[_, AnyConstr, _], +C]
     fromSpecific(this.view.filterKeys(k => !keysSet.contains(k)))
   }
 
+  // The implicit dummy parameter is necessary to avoid erased signature clashes
+  // between this `++:` and the overload overriden below.
+  // Note that these clashes only happen in Dotty because it adds mixin
+  // forwarders before erasure unlike Scala 2.
   @deprecated("Use ++ instead of ++: for collections of type Iterable", "2.13.0")
-  def ++: [V1 >: V](that: IterableOnce[(K,V1)]): CC[K,V1] = {
+  def ++: [V1 >: V](that: IterableOnce[(K,V1)])(implicit dummy: DummyImplicit): CC[K,V1] = {
     val thatIterable: Iterable[(K, V1)] = that match {
       case that: Iterable[(K, V1)] => that
       case that => View.from(that)