The question

I have a client/server application, where the client is an angular-app. The server does provide a couple of typescript-definitions for the API it provides. These definitions are loaded from a project-external folder. So far this worked fine, however now I am upgrading from Angular 7 to Angular 8 (and with that also upgrading Typescript from Version 3.1 to 3.5) and when trying to use a non-static enum from the definitions, this now produces this error:

Module not found: Error: Can't resolve 'tslib' in '/path/to/server/definitions'

When the api.ts-file is located within the client-dir, then everything works just fine.

Why is this happening and what I can I do about it?

Creating a minimal reproducible example

Here is a quick script to create a minimal example to reproduce:

mkdir project
cd project
ng new client --minimal=true --skipGit=true --routing=false --style=css
mkdir server
echo 'export enum ApiSample { X = "X" }' > server/api.ts
sed -i 's/"compilerOptions": {/"compilerOptions": {"paths":{"api":["..\/server\/api"]},/' client/tsconfig.json
sed -i -e "1iimport { ApiSample } from 'api';" -e "s/title = 'client';/title = 'client' + ApiSample.X;/" client/src/app/app.component.ts
cd client
npm start

Or, alternatively, to create an example manually:

Create a new angular project
Create somewhere in the filesystem a file "api.ts" with the content export enum ApiSample { X = "X" }
In the "tsconfig.json" Add {"paths":{"api":["/path/to/server/api"]} to the compilerOptions
Access the ApiSample.X from any file of the angular project
Run npm start

Findings so far

The migrator added "importHelpers": true in tsconfig.json. That is why it worked before and is not working anymore now. Specifying "importHelpers": false in tsconfig.json fixes the issue, but of course this option is there for a reason, so that is no final solution
Explicitly specifying "tslib": ["node_modules/tslib"] fixes the problem. This may be a final solution although it would be interesting to know what is exactly happening and where this is documented.