Code duplication when declaring an ambient class implementing an interface

[knuton]

Ambient class declarations implementing an interface require the user to repeat the type members declared in the interface.

For example with the interface

interface IFoo {
    bar: string;
    qux(): number;
}

an ambient declaration of Foo as

declare class Foo implements IFoo {}

fails with Class 'Foo' incorrectly implements interface 'IFoo'. Property 'bar' is missing in type 'Foo', while the following works as expected, but duplicates all declarations:

declare class Foo implements IFoo {
    bar: string;
    qux(): number;
}

While it should be possible to specialise when declaring an ambient class, it would be very convenient if unchanged type members would not need to be redeclared.

[qc00]

Use declare var Foo: {new(): IFoo}; (per "Class Decomposition" section in the manual)

[knuton]

Thanks, but this is only a partial solution:

Standard classes can be inherited from using extends; decomposed classes cannot.

https://github.com/Microsoft/TypeScript/wiki/Writing%20Definition%20Files#class-decomposition

[mhegazy]

I believe this is a duplicate of #340

@RyanCavanaugh had a good write-up on this issue in #340 (comment)