Unable to assign a boolean argument to a parameter that accepts both true and false

[semyonf]

Bug Report

🔎 Search Terms

function overloads boolean not assignable to parameter of type true false

🕗 Version & Regression Information

v4.4.0-dev.20210601

This is the behavior in every version I tried, and I reviewed the FAQ

⏯ Playground Link

Playground link with relevant code

💻 Code

This will not compile

function inner(flag: true): number;
function inner(flag: false): string;
function inner(flag: boolean): string | number {
    return flag ? 1 : 'A';
}

function outer(flag: true): number;
function outer(flag: false): string;
function outer(flag: boolean): string | number {
    return inner(flag);
}

Unless I throw in a conditional statement that effectively narrows the boolean flag into true and false inside respective branches.

function inner(flag: true): number;
function inner(flag: false): string;
function inner(flag: boolean): string | number {
    return flag ? 1 : 'A';
}

function outer(flag: true): number;
function outer(flag: false): string;
function outer(flag: boolean): string | number {
    if (flag) {
        return inner(flag);
    }

    return inner(flag);
}

🙁 Actual behavior

No overload matches this call.
  Overload 1 of 2, '(flag: true): number', gave the following error.
    Argument of type 'boolean' is not assignable to parameter of type 'true'.
  Overload 2 of 2, '(flag: false): string', gave the following error.
    Argument of type 'boolean' is not assignable to parameter of type 'false'.(2769)

🙂 Expected behavior

Compiles fine

[fatcerberus]

The implementation signature doesn’t count for purposes of overload resolution.

[MartinJohns]

Duplicate of #40827.

[DanielRosenwasser]

You can rewrite as

function inner(flag: true): number;
function inner(flag: false): string;
function inner(flag: boolean): string | number
function inner(flag: boolean): string | number {
    return flag ? 1 : 'A';
}

[behzadmehrabi]

Hi Guyz,
I have the following problem:

function inner(flag: true): number;
function inner(flag: false): string;
function inner(flag: boolean): string | number
function inner(flag: boolean): string | number {
    return flag ? 1 : 'A';
}

// works fine.
let r = inner(true)


function func(b: boolean) {
    return inner(b)
}

// doesn't work as I want it, the return type is "string | number" but it should be "string"
let r2 = func(true)

how can I fix that?

[MartinJohns]

@Bezmehrabi Repeat the overloads on your func function. Also, you better ask such questions on StackOverflow.

[behzadmehrabi]

@MartinJohns Thanks for answering.
Will ask the next one on stackoverflow 👌.

If the inner function is a utility function that's going to be used in so many func like functions, it will be so messy to have too many overloads in app.
Isn't there any other workaround?🤔