Fix scala#11318: add regression test

tests/pos/i11318a.scala

@@ -0,0 +1,4 @@
+extension(a: Int)
+  def b: Int = ???
+  def h: Unit =
+    [A] => (r: Int) => b

tests/pos/i11318b.scala

@@ -0,0 +1,13 @@
+type FunctionK[A[_], B[_]] = [Z] => A[Z] => B[Z]
+type ~>:[A[_], B[_]] = FunctionK[A, B]
+
+trait RepresentableK[F[_[_], _]]:
+  type RepresentationK[_]
+
+  def tabulateK[A[_], C](f: RepresentationK ~>: A): F[A, C]
+
+  extension[A[_], C](fa: F[A, C])
+    def indexK: RepresentationK ~>: A
+
+    def mapK[B[_]] (f: A ~>: B): F[B, C] =
+      tabulateK([Z] => (r: RepresentationK[Z]) => f(indexK(r)))

tests/pos/i11318c.scala

@@ -0,0 +1,6 @@
+extension(a: Int)
+  def b: Int = ???
+  def h: Unit =
+    new Function1[Int, Int] {
+      def apply(r: Int): Int = b
+    }