diff --git a/src/library/scala/collection/Map.scala b/src/library/scala/collection/Map.scala
index 9187c7c04116..c92450718b13 100644
--- a/src/library/scala/collection/Map.scala
+++ b/src/library/scala/collection/Map.scala
@@ -338,7 +338,7 @@ trait MapOps[K, +V, +CC[_, _] <: IterableOps[_, AnyConstr, _], +C]
       case that: Iterable[(K, V1)] => that
       case that => View.from(that)
     }
-    mapFactory.from(new View.Concat(toIterable, thatIterable))
+    mapFactory.from(new View.Concat(thatIterable, toIterable))
   }
 }
 
diff --git a/test/junit/scala/collection/MapTest.scala b/test/junit/scala/collection/MapTest.scala
index 0eda6ccc2769..8d371df6a3b7 100644
--- a/test/junit/scala/collection/MapTest.scala
+++ b/test/junit/scala/collection/MapTest.scala
@@ -42,7 +42,7 @@ class MapTest {
     val m = ListMap(1 -> "one")
     val mm = Map(2 -> "two") ++: m
     assert(mm.isInstanceOf[ListMap[Int,String]])
-    assertEquals(mm.mkString("[", ", ", "]"), "[1 -> one, 2 -> two]")
+    assertEquals(mm.mkString("[", ", ", "]"), "[2 -> two, 1 -> one]")
   }
 
   @deprecated("Tests deprecated API", since="2.13")
diff --git a/test/junit/scala/collection/immutable/HashMapTest.scala b/test/junit/scala/collection/immutable/HashMapTest.scala
index 2365b57a199c..b5a4ff2171f7 100644
--- a/test/junit/scala/collection/immutable/HashMapTest.scala
+++ b/test/junit/scala/collection/immutable/HashMapTest.scala
@@ -1,5 +1,7 @@
 package scala.collection.immutable
 
+import java.util.Collections
+
 import org.junit.Assert._
 import org.junit.{Ignore, Test}
 import org.junit.runner.RunWith
@@ -280,7 +282,7 @@ class HashMapTest extends AllocationTest{
       "i" -> 9,
       "j" -> 10
     )
-    assertSame(nonEmpty2, nonAllocating(nonEmpty1 ++ nonEmpty2))
+    assertSame(nonEmpty1, nonAllocating(nonEmpty1 ++ nonEmpty2))
   }
 
   @Test
@@ -310,4 +312,31 @@ class HashMapTest extends AllocationTest{
     val m3 = m2 ++ m1
     assertEquals(1, m3.apply(2))
   }
+
+  @Test
+  def retainLeft(): Unit = {
+    case class C(a: Int)(override val toString: String)
+    implicit val ordering: Ordering[C] = Ordering.by(_.a)
+    val c0l = C(0)("l")
+    val c0r = C(0)("r")
+    def assertIdenticalKeys(expected: Map[C, Unit], actual: Map[C, Unit]): Unit = {
+      val expected1, actual1 = Collections.newSetFromMap[C](new java.util.IdentityHashMap())
+      expected.keys.foreach(expected1.add)
+      actual.keys.foreach(actual1.add)
+      assertEquals(expected1, actual1)
+    }
+    assertIdenticalKeys(Map((c0l, ())), HashMap((c0l, ())).updated(c0r, ()))
+
+    def check(factory: Seq[(C, Unit)] => Map[C, Unit]): Unit = {
+      val c0LMap = factory(Seq((c0l, ())))
+      val c0RMap = factory(Seq((c0r, ())))
+      assertIdenticalKeys(Map((c0l, ())), HashMap((c0l, ())).++(c0RMap))
+      assertIdenticalKeys(Map((c0l, ())), HashMap.newBuilder[C, Unit].++=(HashMap((c0l, ()))).++=(c0RMap).result())
+      assertIdenticalKeys(Map((c0l, ())), HashMap((c0l, ())).++(c0RMap))
+      assertIdenticalKeys(Map((c0l, ())), c0LMap ++: HashMap((c0r, ())))
+    }
+    check(cs => HashMap(cs: _*)) // exercise special case for HashMap/HashMap
+    check(cs => TreeMap(cs: _*)) // exercise special case for HashMap/HasForEachEntry
+    check(cs => HashMap(cs: _*).withDefault(_ => ???)) // default cases
+  }
 }
diff --git a/test/junit/scala/collection/immutable/TreeMapTest.scala b/test/junit/scala/collection/immutable/TreeMapTest.scala
index 57c1e14c90d1..66d4b7bd7ea1 100644
--- a/test/junit/scala/collection/immutable/TreeMapTest.scala
+++ b/test/junit/scala/collection/immutable/TreeMapTest.scala
@@ -206,7 +206,7 @@ class TreeMapTest extends AllocationTest {
   }
 
   @Test
-  def unionAndIntersectRetainLeft(): Unit = {
+  def retainLeft(): Unit = {
     case class C(a: Int)(override val toString: String)
     implicit val ordering: Ordering[C] = Ordering.by(_.a)
     val c0l = C(0)("l")
diff --git a/test/junit/scala/collection/immutable/TreeSetTest.scala b/test/junit/scala/collection/immutable/TreeSetTest.scala
index 1ff8603b11ec..af29c3518bbe 100644
--- a/test/junit/scala/collection/immutable/TreeSetTest.scala
+++ b/test/junit/scala/collection/immutable/TreeSetTest.scala
@@ -274,6 +274,10 @@ class TreeSetTest extends AllocationTest {
     assertIdenticalElements(Set(c0l), TreeSet(c0l).union(HashSet(c0r)))
     assertIdenticalElements(Set(c0l), TreeSet(c0l).union(TreeSet(c0r)))
 
+    assertIdenticalElements(Set(c0l), HashSet(c0l).+(c0r))
+    assertIdenticalElements(Set(c0l), TreeSet(c0l).+(c0r))
+    assertIdenticalElements(Set(c0l), TreeSet(c0l).+(c0r))
+
     assertIdenticalElements(Set(c0l), HashSet(c0l).++(HashSet(c0r)))
     assertIdenticalElements(Set(c0l), TreeSet(c0l).++(HashSet(c0r)))
     assertIdenticalElements(Set(c0l), TreeSet(c0l).++(TreeSet(c0r)))