interface Number {
    doStuff(): string;
}

interface NotNumber {
    toString(radix?: number): string;
    toFixed(fractionDigits?: number): string;
    toExponential(fractionDigits?: number): string;
    toPrecision(precision?: number): string;
    valueOf(): number;
    doStuff(): string;
}

var x = 1;
var a: Number;
var b: NotNumber;

a = x; 
a = b; 

b = a; 
b = x; 

x = a; // expected error
x = b; // expected error