I've been working on a new TypeScript-like language myself from scratch, which among other things takes this approach. Mine can do some of the numeric stuff shown here, but I'm jealous of (inspired by? :)) some of the crazier stuff going on here, like usage-based argument types and tracing not just value shapes but value reference identities
The "automatic generics" feature in particular is absolutely bonkers. It never even occurred to me that you could do that. I'm wondering if there are unforeseen edge-cases, but also wishing I had it at work. Clunky generics syntax is one of the worst parts of TypeScript, even while generic function types are one of its best parts.
Wow, and side-effects-tracking too! Amazing
I would disagree this is a TypeScript compiler. It's a mostly-typescript-compatible compiler/type-checker.
The Typescript team iterate on it reasonably quickly. If you fall behind as the Typescript team adds new features, and you cannot check that code, is it still "Typescript"?
(This is something that's been possible for decades, but it never hit mainstream before as it's hard to implement it well enough to satisfy the silly preferences of us typical programmers :)).