Typescript error: An outer value of 'this' is shadowed by this container

I had an error in a Typescript class method declaration, but I don't understand how the error message relates back to the bug.

The message seems to be saying that 'this' is of type any, but we are in a class definition, and so I thought 'this' was really clear.

Can someone please explain how the error message relates back to the bug?

Original method:

calcSize = function() {
    return this.width * this.length; // Error on this line
};

// Error text: 'this' implicitly has type 'any' because it does not 
//have a type annotation.ts(2683)
//app.ts(39, 16): An outer value of 'this' is shadowed by this container.

fix:

calcSize() {
    return this.width * this.length;
};

Full context (fixed):

class BaseObject {
    constructor(
        public width: number = 0,
        public length: number = 0
        ) {}

};

class Rectangle extends BaseObject {

    constructor(public width: number = 0, public length: number = 0) {
        super(width, length);
    }

    calcSize() {
        return this.width * this.length;
    };
}

In TypeScript (and ES6) exists two kinds of functions: The classic function declaration and the arrow function. Where the classic function declaration has the default floating binding logic for the this keyword - the arrow function will constantly use the value for this of the context containing the arrow function. In the example this will be the instance of the surrounding class.

class Rectangle extends BaseObject {
  // ..
  calcSize = function() {
    // The keyword "function" will cause "this" to be floating.
    // Since the function is explicitly assigned to calcSize
    // (older) TypeScript may not infer the type of "this".
    // The value of "this" can be re-bound by changing the context
    // using bind or call.
    // -> Value of "this" defaults to the class instance
    return this.width * this.length; // (potential) type Error on this line
  };

  calcSizeAsMember () {
    // This is also a classic function which will use floating binding
    // therefore "this" will be the type of the containing class.
    // The value of "this" can be re-bound by changing the context
    // using bind or call.
    // -> Value of "this" defaults to the class instance
    return this.width * this.length; 
  };

  calcSizeAsArrowFunction = () => {
    // This is an arrow function which has a constantly-bound "this" keyword, 
    // it is not possible to re-bind afterward.
    // The type of "this" is always the type of the containing class.
    // Changing the context using bind or call will have no effect
    // -> Value of "this" is always the class instance
    return this.width * this.length;
  };

};