Fix errors reported under Typescript 2.2.1

[BurtHarris]

Workaround several minor error messages introduced when switching to the Typescript 2.2.1 transpiler. Fixes #285

I'm not sure why my name & sigil are showing up twice in the commits...

[sharwell]

💡 The following equivalent change would not require the type assertion:

getContextEdgeMap(): Map<number, DFAState> {
	let map = new Map<number, DFAState>(this.contextEdges);
	let existing = map.get(-1);
	if (existing !== undefined) {
		if (map.size === 1) {
			let result = new Map<number, DFAState>();
			result.set(PredictionContext.EMPTY_FULL_STATE_KEY, existing);
			return result;
		}
		else {
			map.delete(-1);
			map.set(PredictionContext.EMPTY_FULL_STATE_KEY, existing);
		}
	}

	return map;
}

[sharwell]

💭 Maybe we should check with the TypeScript project about what they expect this code to look like with TypeScript 2.2.

[sharwell]

➡️ The issue is the use of a non-undefined default argument overrides the type annotation, so the compiler has assumed that p cannot be undefined. The solution here (a workaround assuming that this is a bug in the compiler) is assigning p to a local variable at the beginning of the method, and operating on the local variable instead of p itself.

[BurtHarris]

Yes, I tried a local variable and that worked without problem too. I liked this version because it touched less lines of code.

I'm trying to put together an isolated repro case to file a bug with the TypeScript team.

[sharwell]

Yes, I tried a local variable and that worked without problem too. I liked this version because it touched less lines of code.

The form used here is shorter but incorrect. You assert that p._parent is a RuleContext (not undefined), but if that is always the case then the code is an infinite loop.

I'm trying to put together an isolated repro case to file a bug with the TypeScript team.

Here is a self-contained reproducer:

var obj: Context = {
  parent: undefined
};

function working(p: Context | undefined) {
  while (p != null) {
    p = p.parent;
  }
}

function broken(p: Context | undefined = obj) {
  while (p != null) {
    p = p.parent;
  }
}

interface Context { parent: Context | undefined; }

[BurtHarris]

Filed microsoft/TypeScript#14487

[BurtHarris]

The Typescript dev lead's note in above bug suggests they will treat this as a bug. Updated code based on your review.