Additional discussion in [src/index.js].

Graph PNGs were generated from a local Parcel build with the AssertionError caught so that parcel would continue and write graphs.

Bug 1: Difficult DX

The AssertionError is cryptic (refers to Parcel internals) and does not provide any insight.

From the BundleGraph PNG, we can see Parcel knows the problematic foo.css asset is both a "stand-alone" top-level dependency and a dependency below the lazy Foo import.

In this case, it's probably not clear what to do with foo.css -- it is needed at the top level, so it must be emitted there; but it's also needed by the module, so it must be emitted there as well. Perhaps two CSS files, one in each modules, should @import the foo.css content? I am not very familiar with approaches to automatic bundling and CSS imports.

Regardless, if Parcel cannot determine what to do, it could output the asset name and, if dynamic imports can cause a problem, the name of the dynamic module.

Presumably a dynamic import is not the only way a name collision can occur when bundling, but perhaps this case is one parcel can inspect.

Error with or without caching, regardless of multiple runs:

for i in 1 2 3 ; do
  [[ -d dist ]] && rm -rf dist ; npx parcel build --no-cache src/index.js
  # Or
  # [[ -d dist ]] && rm -rf dist ; npx parcel build src/index.js
done

Bug 2: Mis-use of cache / failed cache invalidation

If Parcel has successfully bundled and generated a cache, it will successfully bundle on subsequent runs, even if the current state of the code would not bundle without a cache. It seems parcel is failing to invalidate the cache.

This means it is possible to ship code that will not build during deployment or for teammates, as those builds will not have the local cache help Parcel.

(This will occur with parcel serve as well.)

0. [[ -d .parcel-cache ]] && rm -rf .parcel-cache
1. Run parcel with caching, it will fail: parcel build src/index.js
2. Comment out line 9 import 'Foo/foo.css';.
3. Run parcel with caching again, it will succeed.
4. Uncomment line 9, i.e., revert to step 1, but now parcel has the cache from a successful run.
5. Run parcel with caching... it should fail, but does not.