/**
 * Error for asserting unreachable code paths in TypeScript.
 * Useful for exhaustive matching.
 */
export declare class UnreachableError extends Error {
    constructor(value: never, message?: string, options?: ErrorOptions);
}
/**
 * Beyond terseness, this is useful because `throw` is not an expression,
 * and therefore can't be used in places like Svelte markup without a workaround,
 * at least until this proposal is accepted and widely available:
 * https://github.com/tc39/proposal-throw-expressions
 */
export declare const unreachable: (value: never, message?: string) => asserts value is never;
/**
 * Extract a human-readable message from an unknown thrown value. Returns
 * `value.message` when `value` is an `Error`, otherwise `fallback`
 * (defaulting to `String(value)`).
 */
export declare const to_error_message: (value: unknown, fallback?: string) => string;
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