No name was provided for external module

This is caused because you have an external dependency and you need to declare the name used so that the rollup knows what to look for when building the UMD bundle of my-core.

To fix the warning declare your my-data in ng-package.json

{
  "$schema": "../../node_modules/ng-packagr/ng-package.schema.json",
  "dest": "../../dist/my-core",
  "lib": {
    "entryFile": "src/public-api.ts",
    "umdModuleIds": {
      "my-data": "my-data"
    }
  }
}

I believe this is because since all dependencies are treated as external and your my-data isn't installed through something like npm you need to declare the UMD module id that's expected. See https://github.com/ng-packagr/ng-packagr/blob/master/docs/dependencies.md#resolving-umd-module-identifiers

for anyone else coming here for this error and the above seems tedious to do OR you are just wondering what you changed to make you need this all of a sudden like me.

In my case (in order to make jest work) I had changed the following in tsconfig:

  "compilerOptions": {
    "module": "commonjs",
    "target": "esnext",
  "angularCompilerOptions": {
    "preserveWhitespaces": false
  }

back to:

  "compilerOptions": {
    "module": "esnext",
    "target": "es5"
}

I have no idea why this works, still reading through the rabbit hole provided by the above answerer and I am still trying to get both jest tests to work and my library to build. Anyway, I hope this helps someone.