Alternate syntax for function overloads

Search Terms

class method signature
method signature
function overload syntax
function overload
proxy function
proxy function signature

Suggestion

A new syntax for function overloads:

function Func as FunctionSignature
class A {
  static Func as FunctionSignature
  Func as FunctionSignature
}

This would allow types to be less static, and often more accurate to the purpose of the overloaded function

Right now, we can do something similar using const -

const: FunctionSignature = function() {}

however, the differences in detailed in #25761 and the lack of any equivalent in classes make this a separate issue.

I'm not dead set on the as syntax, but I think it fits with what we already have quite well. The checking done on overloads is similar to that of ({ prop: "HI" } as { prop: number }), and it fits well in and out of classes

It's also worth noting that every use case of this would benefit greatly from #28172

Use Cases

Generic Types

type Pipe<V> = (val: V) => V
function MyFunc as Pipe<string>
// function MyFunc(val: string): string
function MyFunc(...args: any[]): any {

}

Inheritance

function tryInt as typeof parseInt
// function tryInt(s: string, radix?: number): number
function tryInt(...args: [string, number?]) {
  const n = parseInt(...args)
  if (isNaN(n)) throw "Failed to parse"
  return n
}

Combining function implementations

A solution to the problems brought up in #12041 -

function overload as typeof implementation
function overload as typeof otherImplementation
// function overload(num: number): string
// function overload(str: string, num: number): string
function overload(...args: [string, number] | [number]) {
  return args.length === 1 ? implementation(...args) : otherImplementation(...args)
}

Which would work particularly well combined with #28172,

function overload as typeof implementation
function overload as typeof otherImplementation
function overload() {
  return arguments.length === 1 ? implementation(...arguments) : otherImplementation(...arguments)
}

fully checking the types against the current signature of both implementations, and emitting useful errors

Examples

This came up in the context of an argument parser, where I attempt to parse the values using a direct proxy to another method -

class ArgumentIssue {
  confidence: number
}
class Arguments {
  async parse(args: string, cb: (val: string) => any = v => v): Promise<{ [key: string]: any }> {
    // Follows the return value ^
  }
}
class Signature extends Array<Arguments> {
  parse as ArgumentBuilder["parse"]
  // parse(args: string, cb: (val: string) => any): Promise<{ [key: string]: any }>
  parse(a: any, ...args: any[]) {
    let Nearest: [Arguments, ArgumentIssue];
    for (const overload of this) {
      try {
        return overload.parse(a, ...args)
      } catch (issue) {
        if (!(issue instanceof ArgumentIssue)) throw issue;
        if (!Nearest || issue.confidence > Nearest[1].confidence)
          Nearest = [overload, issue];
      }
    }
    throw Nearest;
  }
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

The issue proposes new function and class overload syntax but names no files, tests, or entry points. Start by comparing the proposed declarations with existing overloads and the const-based approach in #25761, then review related discussions in #12041 and #28172. Done would require an accepted design and implementation plan for the type checker.