feat: "Extends" TypeScript util type (#1394)

TypeScript util type "Extends"

resolves #1393

Author: bmingles

packages/utils/src/TypeUtils.ts

@@ -1,3 +1,18 @@
+/**
+ * Util type to create a "subtype" of T. Useful for creating subsets of union
+ * types.
+ *
+ * e.g.
+ * declare type Direction = 'north' | 'south' | 'east' | 'west'
+ *
+ * // This works
+ * type Down = Extends<Direction, 'south'> // 'south'
+ *
+ * // Compiler will complain
+ * type NotDirection = Extends<Direction, 'blah'>
+ */
+export type Extends<T, U extends T> = U extends T ? U : never;
+
 /**
  * Util type to extract the value from an object.
  *