[bakkot]

The type lattice is deliberately conservative; when it encounters a type name it doesn't recognize, like PrivateElement, it assumes that could hold anything and as such it is never an error to assign any value to something of that type.

This heuristic is generally necessary; e.g. a property key is a String or a Symbol, and "property key" is an unknown type, so it is good that ecmarkup does not regard it as an error to assign a String to an unknown type.

However, we don't and (I weakly believe) shouldn't define any types that hold enums. This means we can assume a type name we don't recognize cannot hold an enum, and so assigning an enum specifically to such a type is an error.

[michaelficarra]

I'd rather put the unknown-non-enum case here and leave the final fall-through as unknown. Related, this switch is nearly exhaustive, it just doesn't contain record, which we have a type for. We should add it (either as part of this PR or not, don't care) to make it both more complete and easier to understand.

[michaelficarra]

I'm a bit skeptical that we won't ever want to alias a union that includes an enum, but I'm willing to give it a shot.