This markdown should not produce a link:

[link]: test (()

[link]

The commonmark.js dingus does this right. As the spec sayeth:

https://spec.commonmark.org/0.30/#link-reference-definitions

A link reference definition consists of a link label, optionally preceded by up to three spaces of indentation, followed by a colon (:), optional spaces or tabs (including up to one line ending), a link destination, optional spaces or tabs (including up to one line ending), and an optional link title, which if it is present must be separated from the link destination by spaces or tabs. No further character may occur.

https://spec.commonmark.org/0.30/#link-title

A link title consists of either

• a sequence of zero or more characters between straight double-quote characters ("), including a " character only if it is backslash-escaped, or
• a sequence of zero or more characters between straight single-quote characters ('), including a ' character only if it is backslash-escaped, or
• a sequence of zero or more characters between matching parentheses ((...)), including a ( or ) character only if it is backslash-escaped.

"[+]:\\\n(()" -> [
]

"[+]:\\\n(()" -> [
  Start(Paragraph)
    Text(Boxed("(()"))
  End(Paragraph)
]