Value Definition Syntax parser fails on "<an+b>"

[tidoust]

The following code throws a SyntaxError:

const { definitionSyntax } = require('css-tree');
definitionSyntax.parse('<an+b>');

Error is:

Expect `>`
  <an+b>
-----^

Looking at the Value Syntax Definition, nothing says that such a reference to a data type needs to be only composed of characters that match [a-zA-Z0-9\-], as the parser seems to assume. Granted, that's been the case until now across CSS specifications...

Problem is this construct is now used to define the :nth selector in the CSS Generated Content for Paged Media Module spec in particular, where it is to be interpreted as a reference to the basic data type <an+b>.

[lahmatiy]

The substitution of <an+b> with <an-plus-b> in mdn/data and CSSTree is a practical solution, eliminating the need for parsing exceptions or rule modifications for this infrequently used syntax. While adding an exception or expanding the ident character set are alternatives, adopting <an-plus-b> uniformly ensures parsing consistency and simplicity. However, discussable, I have no strong opinion here.
Interesting fact that the Selectors spec doesn't refer to <an+b> production:

[tidoust]

Yes, we have a workaround in place for Webref data (which gets injected into mdn/data). Now, the single source of truth is supposed to be the spec. Ideally, that source would remain usable. I raised w3c/csswg-drafts#9473 against the definition to suggest <an-plus-b> as an alternative.