Make :is() behave like :matches()

CHANGELOG.md

@@ -3,7 +3,7 @@
 * Deprecate the use of `/` for division. The new `math.div()` function should be
   used instead. See [this page][] for details.
 
-  [this page]: https://sass-lang.com/documentation/breaking-changes/slash-div
+[this page]: https://sass-lang.com/documentation/breaking-changes/slash-div
 
 * Add a `list.slash()` function that returns a slash-separated list.
 
@@ -15,6 +15,10 @@
   made slash-free in both cases. This is a behavioral change, but it's unlikely
   to affect any real-world stylesheets.
 
+* [`:is()`][] now behaves identically to `:matches()`.
+
+[`:is()`]: https://developer.mozilla.org/en-US/docs/Web/CSS/:is
+
 * Fix a bug where non-integer numbers that were very close to integer
   values would be incorrectly formatted in CSS.
 

lib/src/extend/extension_store.dart

@@ -818,9 +818,13 @@ class ExtensionStore {
           // become `.foo:not(.bar)`. However, this is a narrow edge case and
           // supporting it properly would make this code and the code calling it
           // a lot more complicated, so it's not supported for now.
-          if (innerPseudo.normalizedName != 'matches') return [];
+          if (innerPseudo.normalizedName != 'is' &&
+              innerPseudo.normalizedName != 'matches') {
+            return [];
+          }
           return innerSelector.components;
 
+        case 'is':
         case 'matches':
         case 'any':
         case 'current':

lib/src/extend/functions.dart

@@ -21,7 +21,13 @@ import '../utils.dart';
 /// subselectors of their arguments.
 ///
 /// For example, `.foo` is a superselector of `:matches(.foo)`.
-final _subselectorPseudos = {'matches', 'any', 'nth-child', 'nth-last-child'};
+final _subselectorPseudos = {
+  'is',
+  'matches',
+  'any',
+  'nth-child',
+  'nth-last-child'
+};
 
 /// Returns the contents of a [SelectorList] that matches only elements that are
 /// matched by both [complex1] and [complex2].
@@ -734,6 +740,7 @@ bool _selectorPseudoIsSuperselector(
   var selector1 = selector1_; // dart-lang/sdk#45348
 
   switch (pseudo1.normalizedName) {
+    case 'is':
     case 'matches':
     case 'any':
       var selectors = _selectorPseudoArgs(compound2, pseudo1.name);

lib/src/parse/selector.dart

@@ -13,6 +13,7 @@ import 'parser.dart';
 /// Pseudo-class selectors that take unadorned selectors as arguments.
 final _selectorPseudoClasses = {
   "not",
+  "is",
   "matches",
   "current",
   "any",