Optional function argument type mismatch

[d07RiV]

Bug Report

🔎 Search Terms

function optional argument parameter mismatch

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

function foo(x: number, y: number = 1) {
    return x + y
}
type F1 = (x: number) => number
type F2 = (x: number, y?: string) => number

const f1: F1 = foo
const f2: F2 = f1

console.log(f2(1, 'a'))

🙁 Actual behavior

The code compiles without errors/warnings, despite an obvious type mismatch

🙂 Expected behavior

Should probably complain when narrowing function type from F1 to F2

[fatcerberus]

I think this is intentional, as it's equivalent to the (also intentional) type hole that exists with optional properties:

const foo = { x: 777, y: 1206 };
type O1 = { x: number };
type O2 = { x: number, y?: string };

const o1: O1 = foo;
const o2: O2 = o1;
console.log(o2.y);  // oops, not a string

[MartinJohns]

Duplicate of #13043.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.