Add no-duplicate-super

[andy-hanson]

PR checklist

• Addresses an existing issue: Rule request: no-duplicate-super #1983
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update
• Enable CircleCI for your fork (https://circleci.com/add-projects)

What changes did you make?

Added the no-duplicate-super rule, which warns if super() appears twice in a constructor.

[ajafff]

I think this one should be doable. This could result in many false positives in real world code.

The following would be more challenging and I don't know if we want to support code like that:

constructor() {
  if (cond)
    super(true);
  if (!cond)
    super(false);
}

And then there is this nasty but totally valid constructor:

constructor() {
  if(cond)
    return super(foo);
  if(someOtherCondition)
    return super(bar);
  return super(baz);

[andy-hanson]

What if the if has a return instead of else? What if there's a switch statement with break?
TypeScript does control flow analysis, but as far as I can tell, there's no API for accessing it. But I would hate to add a bunch of exceptions that duplicate work done better by TypeScript itself.

[ajafff]

would reword to something like: multiple calls to super() found. It must be called only once.

[nchen63]

I don't think this should go in until it can use control flow analysis. There's going to be a lot of false positives.

[andy-hanson]

Added some minimal control flow analysis. Tell me if there are any test cases I missed.

[andy-hanson]

Not sure why the tests are failing. It only happens in one of the 4 containers.

[nchen63]

Container 3 tests against typescript 2.0.10. I don't know why there aren't any failures in the log, though.

[andy-hanson]

I rebased and it works now.

[nchen63]

lgtm