That's an option, and maybe even the ideal one: an extension which takes a TypeScript+macros file and converts it into TypeScript before feeding to the TypeScript compiler.

However, such a preprocessor will basically need to re-implement TypeScript's type checking. So either TypeScript must expose it via an API, or the preprocessor needs to itself implement a subset (which as mentioned, could also be faster)

This already exists. One example posted elsewhere in these comments is tst-reflect[1], with which you can write this:

    const fooType = getType<Foo>();

Which is exactly what you wanted, just as a plugin to TypeScript rather than built into the official compiler.

[1]: https://www.npmjs.com/package/tst-reflect