test: add in a regression test for #11223

[skip community_build]

closes #11223

Author: ckipp01

tests/pos/i11223.scala

@@ -0,0 +1,21 @@
+object T:
+  trait M[F[_]]
+  trait EE[A, B]
+  trait AE[A]
+  trait DE[A]
+  trait CE[A]
+
+  type A1 = M[AE]
+  type D1 = M[DE] | M[[a] =>> EE[Int, a]]
+  type C1 = M[CE]
+
+  trait F[+R, +A]:
+    def <+>[U, B](b: F[U, B]): F[R | U, A] = null
+    def int: F[R | A1, Int]
+
+  def d1[A](f: => A): F[D1, A] = null
+  def m[R, A](f: F[R | C1, A]): F[R | C1, A] = null
+
+  def x = m { // adding type annotation here helps (m[D1 | A1 | C1, Int])
+    d1(123).int <+> null
+  }