IIRC, type role annotations are definitely needed even if using foreign import data .
Type parameters of nullary data would be phantom but of foreign import data seems to be nominal.
It makes those data types too much restrictive I think.

How about other nullary data in halogen: Halogen.Aff.Driver.State.RenderStateX, Halogen.Query.ChildQuery.ChildQueryBox
These are very internal but the below code is still valid:

test :: forall a b. RenderStateX a -> RenderStateX b
test = coerce

Ah thanks, I had a feeling there was at least one other type I was missing, but my search wasn't bringing it up.

I also need to figure out how to fix this build 😐

I had hit similar issues in some private project about an year ago.
I think this is spago's internal bug regarding to the git integration or something like that.
I personally investigated it but there was no solution, so I switched to spago@next 😞