Readonly<T> not assignable to T with optional properties

[mhegazy]

type T = { a? : number, b: string };
var y: T;
var x: Readonly<T> = y; // Error: Property 'a' is optional

[mhegazy]

Some other related examples:

type T = { a? : number, b: string };
var y: T = {} as any;
var x1: Readonly<T> = y; // Error: Property 'a' is optional
var x2: Partial<Readonly<T>> = y; 
var x3: Readonly<Partial<T>> = y; // Error: Property 'a' is optional
var x4: Partial<T> = y;
var x5: Pick<T, "a"> = y; // Error: Property 'a' is optional