/**
 * A readable reason out of anything that can be thrown, without ever throwing.
 *
 * Two mistakes live here, both made in this codebase before this was shared:
 *
 * An `as Error` cast is a guess. Anything at all can be thrown: a string
 * prints "undefined", and a null throws a TypeError from inside the very
 * handler that exists to keep the program alive.
 *
 * Reading the reason can ITSELF throw. A thrown object can carry a `toString`
 * or `Symbol.toPrimitive` that throws, and the exception then travels straight
 * back out through the recovery path. A formatter that can kill the program it
 * exists to keep alive is the same bug in miniature.
 */
export declare function thrownReason(err: unknown): string;