rust-analyzer/editors/code/src/undefinable.ts

export type NotUndefined<T> = T extends undefined ? never : T;

export type Undefinable<T> = T | undefined;

function isNotUndefined<T>(input: Undefinable<T>): input is NotUndefined<T> {
    return input !== undefined;
}

export function expectNotUndefined<T>(input: Undefinable<T>, msg: string): NotUndefined<T> {
    if (isNotUndefined(input)) {
        return input;
    }

    throw new TypeError(msg);
}

export function unwrapUndefinable<T>(input: Undefinable<T>): NotUndefined<T> {
    return expectNotUndefined(input, `unwrapping \`undefined\``);
}