`asserts` keyword with intersection type is not working

[samchon]

🔎 Search Terms

• asserts
• intersection

🕗 Version & Regression Information

Occurs since v3.7 update, the first update supporting the asserts statement.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/KYDwDg9gTgLgBAMwK4DsDGMCWEVwIYDOBwsAyjFJigOYAUVYSMAXHKgNYoQDuKAlK0LFYBOAyZjRBClWpwAZHGmUacAN4AoONrEI4tGAE8wwCHvHwAhAF5rcAOTLZ9vlp3uYACyg84KYNxwAKJQPlC09gCSKIzwmKJc8HhKMjQuANxu2lDAMEhQuBaZAL5AA

💻 Code

export function assertString(input: unknown): asserts input is string & string {
    if (typeof input !== 'string')
        throw new Error('Input is not a string');
    return input;
}

🙁 Actual behavior

It is possible to declare an asserts guard function with intersection return type. There's no error until that.

However, the intersection return type does not work, and just working as a void return type.

Type 'string' is not assignable to type 'void'.

🙂 Expected behavior

Hope the intersection return type to work.

Additional information about the issue

No response

[MartinJohns]

The type string & string is the same as string.

It's not clear what you actually want and how it should work. Assertion functions don't return anything. You're just declaring that whatever input is is of the type string & string (which is equal to string).

If I had to guess I'd say you want #40562, and just mistakingly throw unrelated terms together (the intersection type).

Regardless, the error is correct. There is no bug here.

[samchon]

Got it, what I want is to making a function returning T value. Exact type maybe (asserts input is T) & T.

[MartinJohns]

That would be a feature request, not a bug report, and it already exists: #40562

Currently such functions are not possible.

[samchon]

Thanks for fast response. I should consider another way.