Implement Selectors Level 4: support complex selectors inside :has(), :is(), :where(), and selector-accepting pseudo-elements

[Copilot]

The selector parser treated pseudo-class function arguments (:has(), :is(), :where(), etc.) as simple expressions (an+b-style), rejecting attribute selectors and pseudo-classes inside them. Additionally, CSS4 pseudo-elements that accept selector arguments (::slotted(), ::cue(), ::cue-region()) were not supported.

# Previously failed:
fieldset:has(option[value="foo"]:checked) {}
fieldset:has(option:checked) {}
x::slotted(:is(div)) {}
x::slotted(.foo:hover) {}
# Already worked:
fieldset:has(option) {}

Changes

• Constants.selector_pseudos — new frozenset of Level 4 pseudo-classes that accept full selector lists: :has(, :is(, :where(, :matches(, :any(

• Constants.selector_pseudo_elements — new frozenset of CSS4 pseudo-elements that accept full selector arguments: ::slotted(, ::cue(, ::cue-region(

• _pseudo() — detects selector-accepting pseudo-classes and pushes a pseudo-class-has context returning simple_selector_sequence (which permits attrib, pseudo, HASH, class) instead of the restricted expressionstart; also handles selector_pseudo_elements by pushing a pseudo-element context with simple_selector_sequence

• _ident() — handles pseudo-class-has context by treating identifiers as type-selectors, returning simple_selector_sequence2 + combinator to allow compound selector chaining

• _S() — spaces inside pseudo-class-has when a combinator is expected are treated as descendant combinators, enabling :has(div span)

• _char() — new ) handler for pseudo-class-has context to properly close the function and restore outer selector state; fixes the return value when closing pseudo-class-has inside a pseudo-element context (e.g. ::slotted(:is(div))); adds a handler to close selector-accepting pseudo-elements (e.g. ::slotted(.foo))

[jaraco]

@copilot Add a newsfragment (towncrier style).

[Copilot]

@copilot Add a newsfragment (towncrier style).

Added newsfragments/66.feature.rst in commit b7efeec.

[Copilot]

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[jaraco]

@copilot During your investigation on the coverage issue, you observed that the uncovered code was modeled after other parts of the code that lacked coverage. Please review the PR to ensure that there's no duplication introduced by the proposed change (that any common behaviors are refactored to be shared instead of copied); there may be none.

[Copilot]

@copilot During your investigation on the coverage issue, you observed that the uncovered code was modeled after other parts of the code that lacked coverage. Please review the PR to ensure that there...

Reviewed and found two instances of duplication, both now eliminated in c3a084d:

1. _pseudo(): The two if-blocks that both called self.context.append(...) then return Constants.simple_selector_sequence (one for selector_pseudos, one for selector_pseudo_elements) were merged into a single if/elif/else with one shared append+return.

2. _char(): The separate pseudo-element + combinator in expected closing block (which had the same append/pop/check structure as the existing expression-closing block) was removed. Its trigger condition was folded into the existing block's condition as an or clause. A comment was added to clarify that context is intentionally checked pre-pop — it identifies the type of pseudo that was just closed (element vs class), which governs what tokens may follow.

[jaraco]

@copilot Remove .gitignore and LICENSE; These concerns are managed outside the repo.

[Copilot]

@copilot Remove .gitignore and LICENSE; These concerns are managed outside the repo.

Removed both files in commit 2e7a0c4.