Suggestion: Provide type information to property decorators

[PyroVortex]

Issue
Currently there is no way to enforce type-safety on property decorators.

Current property decorator syntax

function defaultValue<T>(value: T) {
    // No way to enforce that target[propertyKey] is of type T
    return function (target: any, propertyKey: string) {
        Object.defineProperty(target, propertyKey, { value: value });
    };
}

class Foo {
    @defaultValue(4)
    x: number; // Compiles without error

    @defaultValue('foo')
    y: number; // Also compiles without error, no current way to enforce type safety
}

Proposed property decorator syntax

function defaultValue<T>(value: T) {
    return function (target: any, propertyKey: string, descriptor: TypedPropertyDescriptor<T>) {
        // Note that descriptor has the value (<TypedPropertyDescriptor<T>>undefined)
        Object.defineProperty(target, propertyKey, { value: value });
    };
}

class Foo {
    @defaultValue(4)
    x: number; // Compiles without error

    @defaultValue('foo')
    y: number; // Error: argument of type 'TypedPropertyDescriptor<number>' is not
               // assignable to parameter of type 'TypedPropertyDescriptor<string>'
}

The descriptor parameter could really be anything; its only purpose is to provide the compiler with type information about the property being decorated. I used TypedPropertyDescriptor<T> here for consistency with method and accessor decorators. Current downlevel (ES5) emit already passes undefined explicitly as the third parameter to the decorator function.

[mhegazy]

Property decorators do not get PropertyDiscriptor's like method decorators do. that would be the way to pass the type to the function.

The reason this does not happen is that these properties are not defined until the constructor runs, so the value can not be computed ahead of time as it is an expression and can have side effects.

It is possible that we can do this when Public class fields TC39 proposal is ratified. this will give us some standard framework to represent initialization of a property that will run at construction time.