This implements support for using instanceof as discriminator in untagged variants. This opens up for "natively" supporting using a bunch of built in things together, which isn't possible right now because we mostly rely on typeof:

• Promise
• Date
• RegExp
• File
• Blob
• BigInt This is a typeof, add in separate PR
• FormData I was sure I saw one, but right now can't find an API that returns FormData together with some other type. So waiting with this one.

Adding more to this list as we go.

One restriction here is that we need anything using instanceof to be backed by actual types defined in the compiler. promise<_>, Js.Re.t, Js.Date.t all fit the bill well here. We can add simple abstract types for the other relevant things like File and Blob. Libs can then reference those types for themselves, and be compatible with untagged variants.

Follow up work from this:

• Look into optimizing output when there are literal checks.
• Dig through aliases so using aliased types work.

Dig through aliases so using aliased types work.

Will it work even with opaque types? Sounds like a magic 🙌

Dig through aliases so using aliased types work.

Will it work even with opaque types? Sounds like a magic 🙌

Depends on what you mean. It'll work with a few select built in opaque types, but not any opaque type. Aliases is about if you for example have type fn = unit => string and want to use fn as payload. Or the Dict.t case from RescriptCore, which should work because Dict.t = Js.Dict.t but right now doesn't because we don't follow aliases. I think anyway, I haven't investigated this deeply yet.