I have a problem with the following code when trying to create a proxy to a function with many overloads:

// The function to proxy
function original (a: number): boolean;
function original (a: string): boolean;
function original (a: boolean): boolean;

function original (a: number | string | boolean): boolean {
  return true;
}

// The proxy
function pass (a: string | number){
  return original(a); // here
};

pass(5);
pass('a');
pass(true);

When trying to create a proxy to the original function TS throw me the following error :

No overload matches this call.
  Overload 1 of 3, '(a: number): boolean', gave the following error.
    Argument of type 'string | number' is not assignable to parameter of type 'number'.
      Type 'string' is not assignable to type 'number'.
  Overload 2 of 3, '(a: string): boolean', gave the following error.
    Argument of type 'string | number' is not assignable to parameter of type 'string'.
      Type 'number' is not assignable to type 'string'.
  Overload 3 of 3, '(a: boolean): boolean', gave the following error.
    Argument of type 'string | number' is not assignable to parameter of type 'boolean'.
      Type 'string' is not assignable to type 'boolean'.(2769)
input.tsx(5, 10): The call would have succeeded against this implementation, but implementation signatures of overloads are not externally visible.

Few things to know about this :

• I reduced my bug to this minimal situation where the problem happens
• The pass(true) call return an error, so typing seems to work for the pass function
• In my situation, the original function is from a module, so if any situation exist without modifications on it would be perfect

Here is a TS Playground with the previous code.

CodePudding user response：

If getting rid of the error is your objective, then checking the type before calling the original function will get rid of the error.

function pass (a: string | number){
  if (typeof a === "number") {
    return original(a);
  } else if(typeof a === "string"){
    return original(a);
  }
  throw new Error(`Expected string or number`);
};

CodePudding user response：

Just cast it to needed type

function original(a: number): boolean;
function original(a: string): boolean;
function original(a: boolean): boolean;

function original(a: number | string | boolean): boolean {
  return true;
}

const pass = original as (a: string | number) => boolean

pass(5);
pass('a');
pass(true);

https://www.typescriptlang.org/play?ssl=15&ssc=1&pln=1&pc=1#code/GYVwdgxgLglg9mABHATjA5jMBDANgCmwC5EwQBbAIwFMUBKEyuOXa7MAbgFgAoUSWAmRpMOAsUQBnKGjDoGiJizade-aPCSoMWPIUbNW7BUqOqea8BqHbReiWSq1EAHykys6V4sMqTv9kQAb15ERBRqKBAUJBkQam4eAF9eXggEaUQAB2xJSUQAXmEdMURcxH13WS83Rxp6QoA H2V2VJ4cvPwAVjpEzsl8AHJsIb7eAfw46nGLHiA