Add support for extraFileExtensions on WatchCompilerHost

[sheetalkamat]

To support typescript-eslint/typescript-eslint#1813

[DanielRosenwasser]

So just to be clear, there's a break in the API which needs to be documented - but any consumer that updates its usages of this function should be fixed?

[DanielRosenwasser]

If so, can somebody link me to the thing that caused the original break?

[uniqueiniquity]

#37278 removed the function that was being used. I believe it was internal.

[DanielRosenwasser]

In that thread @JoshuaKGoldberg removed something that appeared unused. If removing it affected things, then I assume it really was used. Can someone explain what happened step-by-step?

[uniqueiniquity]

From my perspective, Brad tagged me in typescript-eslint/typescript-eslint#1813, I looped in Sheetal, and she quickly identified the issue and put up a fix.

@sheetalkamat Could you give us a sense of what happened regarding Josh's PR?

[sheetalkamat]

@DanielRosenwasser There was onCachedDirectoryStructureHostCreate method on WatchCompilerHost which was internal (intentionally) that we never re;lied on and hence was removed (the unused variable host was set in that method callback)` Given this was internal method, no body shouldnt have been relying on that but that was not the case and hence the break for eslint. I believe PR #37278 is correct though in removing those things.