Custom type error messages in TypeScript

example.ts

declare const StaticErrorTag: unique symbol;
type StaticError<E extends string> = { [StaticErrorTag]: E }

type AAndBAreSame<T> = T extends {a: any}
    ? T extends {b: T["a"]}
    ? T
    : StaticError<"Types of `a` and `b` differ">
    : StaticError<"No `a` field">

const f = <T,>(t: T & AAndBAreSame<T>) => {};

f({a: 5, b: 10});