Use NodeFlags.Ambient (#17831)

Is it possible that declarations.length > signatures.length but we don't want to add an implementation?
While I'm here, TypeChecker.signatureToSignatureDeclaration has a non-nullable result. Then in signatureToMethodDeclaration it's cast to another non-nullable type. Then it's tested for existence!

Is it possible that declarations.length > signatures.length but we don't want to add an implementation?

Good catch, but I don't even understand what that check is for and what it's doing. @aozgaa?

See this comment (a few lines above): https://github.com/Microsoft/TypeScript/pull/19708/files#diff-6d6034083b71c2ed75493db0fe725ec5R88.

An example (haven't checked by actual debugging) might be

class C {
     foo(x: number): void;
     foo(x: string): void;
     foo(x: number | string) { console.log(x); }
}

declare class D implements C {
}

Then I think adding foo to D would be an example.

I think this check should be inside createMethodImplementingSignatures, which will just omit the body if we are in an ambient context.

EDIT: no, we do not need an implementation signature in this case. Doh.

It's wierd to create an optional method with a body even if that's legal.
If the method is optional, though, we don't want to make it look like it must be implemented; maybe just add an // optional, delete if you don't want to implement this comment and let the user decide?

We might as well give them the return type from the signature, right?

Technically it's unsound to union the return types in most cases, but we could do that in a different PR.

Intersection instead? It's better that they get an error when trying to return a value from the method, than that they get an error that the class failed to implement the interface, which are always hard to read..

The intersection will be impossible to fulfill most of the time.

function foo(x: number): number;
function foo(x: string): string;
function foo(x: string | number): ??? {
  // An intersection is impossible to return
}

All I'm saying is that it's unsound to do the union, but we do allow it because it's just convenient. You're right that we should probably still do it. Just felt weird.