Expand existing custom selectors in selector definition about postcss-custom-selectors

[MoOx]

Does the spec allow that ? If so please make a PR :)

[neatonk]

It's not mentioned in the spec. That said, the @custom-selector grammar accepts a <selector> and allows the provided <custom-selector> to be used as a <selector> even though the grammar doesn't include it yet. It seems reasonable then to say that the <selector> in the @custom-selector grammar should accept any <custom-selector> as well.

I'm happy to put together a pull request if this sounds reasonable enough.

[MoOx]

We can ask @tabatkins for that since he is the author of the spec ;)

[neatonk]

Good call.

[tabatkins]

Note that custom selectors are pseudo-classes, so it would be :--heading, etc.

But yeah, you can totes use custom pseudo-classes in the definitions of other custom pseudo-classes. You need to run a cycle detector, though, and mark all of the ones that are involved in the cycle as invalid (the rules themselves are valid, but they define a pseudo-class that matches nothing). This hasn't been put into the spec yet, but it matches what custom properties do.