Gentype: handle null/nullable/undefined from Pervasives

[cknitt]

No description provided.

[cristianoc]

This looks great.
Before merging, or perhaps after: we should settle on where these types exist.

Having them exist in 3 or 4 places is an opportunity for issues down the line in tooling (editor, hovering) and compiler updates.

What's the ground truth here?
I guess for each type, we should have a preferred way of expressing that type, at least.
Ideally, a unique way, but that seems difficult to achieve while maintaining backwards compatibility.

Thoughts @zth ?

[cknitt]

What's the ground truth here? I guess for each type, we should have a preferred way of expressing that type, at least. Ideally, a unique way, but that seems difficult to achieve while maintaining backwards compatibility.

If we follow my current approach in #7129, the ground truth would be in the CoreTypes module.
Directly in Pervasives won't work because of circular dependency issues.

In any case, we still have to be able to handle the different aliases.

[cknitt]

Will keep this open until #7129 is done, as we'll run into the same issue with any other types we alias in Pervasives (e.g. date).