Design Meeting Notes, 10/18/2024

[DanielRosenwasser]

Conditional Type Narrowing

#56941

• Shouldn't be any perf impact for existing code because most code doesn't actually use a conditional type as the return type of a function, or it just casts.

• It is a semantic check on the return type (not syntax - you can use type aliases).

• If we want to support this kind of narrowing, we need to dive into the conditional expression and check each branch separately.

  • Original PR did this for every function, caught bugs - but maybe we generalize there later on. For now, the current check is only when checking against a a conditional return type.

• How does this work with contextual types?

  function makeEquals<T extends string | number>(x: T):
      T extends string
          ? (x: string) => boolean
          : (x: number) => boolean {
  
      // are these well-typed?
      return typeof x === "string" ? (y => x === y) : (y => x === y);
  }
  

  • Currently it does work. See PR for details.

• Seems like no objections to current design - plan for 5.8.