overloading_generics_and_runtime_types.ts

class Checkpoint<Semantic> {
   type: 'Script' | 'Statement'

   constructor(blah: any, type: 'Script'): Checkpoint<Script>
   constructor(blah: any, type: 'Statement'): Checkpoint<Statement>
   constructor(blah: any, type: 'Script' | 'Statement') {
      // ...
      this.type = type
   }

   producesScript(): this is Checkpoint<Script> {
      return this.type === 'Script'
   }

   producesStatement(): this is Checkpoint<Script> {
      return this.type === 'Statement'
   }

   f<T>(callback: (val: Semantic) => T): T {
      // ... now do runtime type-checks, and narrow knowledge of the resultant type from `callback`
   }
}