Compile time comparison of accessors fails to differentiate on setter/getter

Bug Report

🔎 Search Terms

• type check read-only accessor
• compile time check for setter

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type checking of accessors

⏯ Playground Link

Playground link with relevant code

💻 Code

class Immutable {
    get prop(): string {
        return 'foo';
    }
}

class Mutable {
    set prop(value: string) {}
}

class Other {
    get prop2(): string {
        return 'foo';
    }
}

const test = (v: Mutable) => {};

test(new Other());
//Compile error as expected
test(new Immutable());
//No error, but would also expect one

🙁 Actual behavior

Other correctly causes a compile error as it does not have prop property. Immutable passes compiler check even though the prop property is read-only (just a get)

🙂 Expected behavior

I would expect both Other and Immutable to fail the typing of test as neither adhere to the structure of Mutable.

If inside test I try and set v.prop when Immutable is passed in I get a runtime error. I would expect the typing to guard against this.

[fatcerberus]

This isn’t specific to accessors and happens with readonly properties as well. Unfortunately readonly-ness is a very weak guarantee in TS :(

[mkantor]

The other issue is the absence of a writeonly modifier to model properties with setters but no getters. For example, this would ideally be a compile-time error:

const x = { set foo(_x: string) {} }
x.foo = 'ok' // should be allowed
x.foo.includes('yikes') // should not be allowed

[RyanCavanaugh]

Duplicate #21759 / #18770

[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.