compiler throws a false positive on interface conformance

[bcherny]

is there a better way to express this, without explicitly listing out every member of the interface?

interface Foo {
    a: string
}

// ok
class Bar implements Foo {
    constructor (public a: string) {}
}

// Error: Class 'Baz' incorrectly implements interface 'Foo'.
//        Property 'a' is missing in type 'Baz'
class Baz implements Foo {
    constructor (a: Foo) {
        Object.assign(this, a)
    }
}

demo: http://www.typescriptlang.org/Playground#src=interface%20Foo%20%7B%0A%09a%3A%20string%0A%7D%0A%0A%2F%2F%20ok%0Aclass%20Bar%20implements%20Foo%20%7B%0A%09constructor%20(public%20a%3A%20string)%20%7B%7D%0A%7D%0A%0A%2F%2F%20Error%3A%20Class%20'Baz'%20incorrectly%20implements%20interface%20'Foo'.%0A%2F%2F%20%20%20%20%20%20%20%20Property%20'a'%20is%20missing%20in%20type%20'Baz'%0Aclass%20Baz%20implements%20Foo%20%7B%0A%09constructor%20(a%3A%20Foo)%20%7B%0A%09%09Object.assign(this%2C%20a)%0A%09%7D%0A%7D

[RyanCavanaugh]

There isn't a better way to do this. See also #340 and many duplicates that reference it.

[jeffreymorlan]

@RyanCavanaugh: In TypeScript 1.7+ it seems like you can do this with a merged class/interface declaration:

interface Baz extends Foo {}
class Baz {
    constructor (a: Foo) {
        Object.assign(this, a);
        console.log(this.a); // no error here
    }
}

[RyanCavanaugh]

@jeffreymorlan you're very right! I forgot about that

[bcherny]

thanks @jeffreymorlan! still not pretty, but it's an improvement.