Fix #12944: Add regression test

Author: OlivierBlanvillain

tests/pos/12944/12944_1.scala

@@ -0,0 +1,11 @@
+object Test1 {
+  type ++[L, R] = (L, R) match
+    case (Int, Int)       => 2
+    case (String, String) => "2"
+    case (String, Int)    => "2"
+    case (Int, String)    => "2"
+
+  type Bar[W <: Int] = W ++ "" ++ W
+
+  val works = summon[Bar[2] =:= "2"]
+}

tests/pos/12944/12944_2.scala

@@ -0,0 +1,5 @@
+import Test1._
+
+object Test2 {
+  val fails = summon[Bar[2] =:= "2"]
+}