Why index signature type should be assignable to all known members?

[mohsen1]

I want to define an object type where I explicitly define types of known members and then have a different type for unknown members (type of the index signature):

interface Known {
    foo?: () => boolean;
}

interface Unknown {
    [k: string]: () => number;
}

type All = Known & Unknown;

let a: All = {
    foo: () => true
}

let b: All = {
    randomKey: () => 1
}

With above code, TypeScript complains that "Property foo is incompatible with index signature".

**Why type of property foo has to be compatible with index signature?

This issue originally came up here: typescript-eslint/typescript-eslint#272 (comment)

[RyanCavanaugh]

Why type of property foo has to be compatible with index signature?

That's what an index signature means - it applies to all properties, not just those not declared. See #17867 and related issues for more discussion.