It all started when @arcanis raised a PR which added Yarn PnP support to ts-loader. To quote the PR:

This PR adds a new resolveModuleName option that can be used to help TypeScript figure out the location of the files on the disk. It mimics the TypeScript API.

#862

Time passed and it was all good. Then the wonderful @arcanis submitted a very similar PR to the fork-ts-checker-webpack-plugin:

TypeStrong/fork-ts-checker-webpack-plugin#250

This PR was very similar to the ts-loader one. However, as I was reviewing it I noticed something. TypeStrong/fork-ts-checker-webpack-plugin#250 (comment)

To quote myself:

Thanks! I was just looking at the work you did on ts-loader: #862

You only added resolveModuleName there.

This adds 2 options: resolveModuleNameModule and resolveTypeReferenceDirectiveModule

Does ts-loader need a resolveTypeReferenceDirective to work with pnp as well? Or is it not necessary?

And @arcanis response:

I think that might be useful yep. When I initially made the PR for ts-loader I wasn't aware of what resolveTypeReferenceDirective was used for (/// <reference types="...">) - I just discovered it when implementing the feature for fork-ts-checker-webpack-plugin and only because create-react-app uses it to automatically define the *.svg typings.

So I decided to add the corresponding functionality to ts-loader; I gave it its own resolveTypeReferenceDirective:

#921

Now up until this point, when we created a ServicesHost in ts-loader we didn't actually supply resolveTypeReferenceDirective at all - in fact it was commented out in the codebase as you can (slightly embarrassingly) see here:

https://github.com/TypeStrong/ts-loader/pull/921/files#diff-70b0dbb6219cb6164e67ffb54a8f23c0L140

And it turned out that introducing it at all has had a breaking side effect for jest users:

#919 (comment)

Clearly this issue could be "fixed" by just dropping the resolveTypeReferenceDirectives from ts-loader. But if we do that, then I'm assuming we break Yarn PnP support too (only for type reference directives that is).

Where I am is here: I can no longer remember the reason I commented out supplying resolveTypeReferenceDirectives. Flat out, I just don't remember. And so we finally get to the question:

Is ts-loader using this API correctly? Should we be using differently? Or is it actually not to be used? I'm after guidance. Please educate me!

cc @andrewbranch