Fix failed builds when using thread-loader

[valerio]

Possible fix for #1206

When using thread-loader, the loader._compiler field is undefined, this causes a runtime error when trying to use it as a key in a WeakMap.

This PR changes the instance cache so that it now uses a dummy "marker" object as reference when the compiler instance is undefined.

[johnnyreilly]

This looks great! Thank you!

Now I must reveal a prejudice of mine... I don't really like classes. It's not an accident that ts-loader is pretty much a class free codebase.

From the looks of it, class InstanceCache is a class that is instantiated just the once. I wonder if it would be straightforward to switch over to the revealing module pattern here perhaps?

No worries if you don't have time now, this looks functionally completely correct.

[valerio]

From the looks of it, class InstanceCache is a class that is instantiated just the once. I wonder if it would be straightforward to switch over to the revealing module pattern here perhaps?

Fair point! Switching to that should be fairly easy, I'll make the change 👌

[johnnyreilly]

Awesome - you want to include the package.json and CHANGELOG.md changes as well? All good and we'll ship!

[valerio]

Awesome - you want to include the package.json and CHANGELOG.md changes as well? All good and we'll ship!

Done!

I'll also take a look at the other error that I saw when using thread-loader and open an issue for it (unless there's something related already).

[valerio]

@johnnyreilly I've unlinked this PR from the original issue (#1206), so that it can be closed after users report that it's fixed, but feel free to close it if you prefer handling it that way.

[johnnyreilly]

Travis is stuck - going to override and merge.

[johnnyreilly]

v8.0.9 should be out in the next 20 mins or so - thanks for the quick turnaround @valerio!

https://github.com/TypeStrong/ts-loader/releases/tag/v8.0.9