test: add in regression test for #7653 (#17474)

Closes #7653

Author: nicolasstucki

tests/pos/i7653.scala

@@ -0,0 +1,22 @@
+// https://github.com/lampepfl/dotty/issues/7653
+
+object options2 {
+  type Option[T] = {
+    def isEmpty: Boolean
+  }
+  type None[T] = Option[T]
+  val none: () => Option[Nothing] = () =>
+    new {
+      def isEmpty = true
+    }
+  val mkNone0: [T] => () => Option[Nothing] = [T] =>
+    () =>
+      new {
+        def isEmpty = true
+    }
+  val mkNone: [T] => () => Option[T] = [T] =>
+    () =>
+      new {
+        def isEmpty = true
+    }
+}