Generate :is() within :is() and :has()

CHANGELOG.md

@@ -10,6 +10,16 @@
 * **Breaking change:** A variable definition may now only have a single
   `!global` flag or `!default` flag, or one of each.
 
+* **Potentially breaking bug fix:** In some cases involving a `:has()` selector
+  with a leading combinator being `@extend`ed by a selector that contained its
+  own combinators, `@extend` used to generate incorrect selectors. This has been
+  fixed.
+
+* When extending selectors in `:is()`, `:has()`, or `:where()`, `@extend` will
+  now generate `:is()` selectors rather than manually combining selectors, since
+  there are no browser compatibility concerns in that context. This fixes
+  several related extension bugs in this context.
+
 ### Bogus Combinators
 
 * **Breaking change:** Selectors with more than one combinator in a row, such as

lib/src/ast/selector/list.dart

@@ -63,6 +63,16 @@ final class SelectorList extends Selector {
   /// [relative selector]: https://www.w3.org/TR/selectors-4/#relative-selector
   bool get isRelative => components.every((complex) => complex.isRelative);
 
+  /// If this selector list is composed of a single complex selector, returns
+  /// it.
+  ///
+  /// Otherwise, returns null.
+  ///
+  /// @nodoc
+  @internal
+  ComplexSelector? get singleComplex =>
+      components.length == 1 ? components.first : null;
+
   /// Throws a [SassException] if `this` isn't a CSS selector that's valid in
   /// various places in the document, depending on the arguments passed.
   ///

lib/src/ast/selector/pseudo.dart

@@ -119,6 +119,14 @@ final class PseudoSelector extends SimpleSelector {
         normalizedName = unvendor(name),
         super(span);
 
+  /// A shorthand for creating an `:is()` selector.
+  ///
+  /// @nodoc
+  @internal
+  factory PseudoSelector.isSelector(
+          Iterable<ComplexSelector> components, FileSpan span) =>
+      PseudoSelector('is', span, selector: SelectorList(components, span));
+
   /// Returns whether [name] is the name of a pseudo-element that can be written
   /// with pseudo-class syntax (`:before`, `:after`, `:first-line`, or
   /// `:first-letter`)

lib/src/extend/empty_extension_store.dart

@@ -55,4 +55,6 @@ final class EmptyExtensionStore implements ExtensionStore {
         EmptyExtensionStore(),
         {},
       );
+
+  void trimModernSelectors() {}
 }