Suggestion: readonly method

Problem:

A method (function written with the ES6 method syntax) can currently be replaced with another function by a simple assignment.

As an example consider the following snippet of code:

class Person {
    constructor(...) {...}
    displayName (): string {...}
}
const x: Person = new Person(...)
x.displayName = function(this: Person): string {...} // Assignment

Because the readonly modifier is not usable for methods, this is not possible to prevent this kind of assignments.

Proposal:

A method is always readonly.

The following codes are identical:

class Person {
    displayName (): string {...}
}

class Person {
    readonly displayName: (this: Person) => string = function () {...}
}

Compatibility:

This is a breaking change. However, method syntax is recent and mostly used in classes. Codes which assign a function to a method are certainly rare.

Temporary workaround:

Do not use method syntax in your classes and interfaces. Note hat this leads to very verbose codes.

If you use an interface, the verbosity is acceptable. However you get also strict variance.

interface PersonI {
    readonly displayName: (this: Person) => string
}

No source file or test is named. Start by comparing the class method and readonly property examples in the issue, then trace where TypeScript checks assignments to methods. Done would require a decided readonly-method behavior, compatibility handling for the proposed breaking change, and tests covering assignment rejection.