Why doesn't an implementation use the types on its parameters from its interface?

[HomenSimpsor]

interface Bank {
    balance: number;
    withdraw(x: number): number;
}

class PowellCountyBank implements Bank {
    balance: number = 0;

    withdraw(x) {
        return (this.balance - x);
    }
}

var b = new PowellCountyBank();

console.log(b.withdraw({}));
                     // ^ Expect type error here.

(Playground)

The implementation shows the return type of withdraw but not type for the parameter:

[ahejlsberg]

Duplicate of #3667. Fix available in #6118.

[ahejlsberg]

Actually, #6118 doesn't affect methods, only property initializers. Reopening as suggestion.

[sandersn]

Commit 055ae8c from #6118 adds method contextual typing as well. Here's a cut-down sample from the test (it works for extending classes, too):

interface Ringer {
    ring: (times: number) => void;
}
class Alarm implements Ringer {
    str: string;
    ring(times) {
        this.str = times; // error
    }
}