Reports

For a more general solution that works in all cases, check out this trick:

type Untag<T, Tag> =
  ((_: FlipVariance<T>) => never) extends (
    (_: FlipVariance<infer T & Tag>) => infer T
  ) ?
    T
  : never;
interface FlipVariance<T> {
  (_: T): void;
}

type TestResult = Untag<string & { _tag: "foo" }, { _tag: "foo" }>; // => string

This is made possible by TypeScript’s special support for intersection type inference in generics, allowing functions like untag<T>(a: T & { tag: any }): T to work.

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