E0321: expected '?' to mark tuple element as optional

TypeScript tuple types can contain optional elements. These optional elements must be last. It is a syntax error to write a required tuple element after an optional one:

function parseVersion(
): [number, number?, string] {
  /* ... */
}

To fix this error, mark the tuple element as optional by adding ?:

function parseVersion(
): [number, number?, string?] {
  /* ... */
}

Alternatively, use a union type:

function parseVersion(
): [number, number?]
 | [number, string]
 | [number, number, string] {
  /* ... */
}

Introduced in quick-lint-js version 2.10.0.

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