Improve unification of :host and :host-context (#1471)

See sass/sass#3134

Author: nex3

CHANGELOG.md

@@ -1,3 +1,10 @@
+## 1.40.2
+
+* The `selector.unify()` function now correctly returns `null` when one selector
+  is a `:host` or `:host-context` and the other is a selector that's guaranteed
+  to be within the current shadow DOM. The `@extend` logic has been updated
+  accordingly as well.
+
 ## 1.40.1
 
 * **Potentially breaking bug fix:** `min()` and `max()` expressions outside of

lib/src/ast/selector/pseudo.dart

@@ -5,6 +5,7 @@
 import 'dart:math' as math;
 
 import 'package:charcode/charcode.dart';
+import 'package:meta/meta.dart';
 
 import '../../utils.dart';
 import '../../visitor/interface/selector.dart';
@@ -47,6 +48,15 @@ class PseudoSelector extends SimpleSelector {
   /// This is `true` if and only if [isSyntacticClass] is `false`.
   bool get isSyntacticElement => !isSyntacticClass;
 
+  /// Whether this is a valid `:host` selector.
+  @internal
+  bool get isHost => isClass && name == 'host';
+
+  /// Whether this is a valid `:host-context` selector.
+  @internal
+  bool get isHostContext =>
+      isClass && name == 'host-context' && selector != null;
+
   /// The non-selector argument passed to this selector.
   ///
   /// This is `null` if there's no argument. If [argument] and [selector] are
@@ -125,9 +135,20 @@ class PseudoSelector extends SimpleSelector {
   }
 
   List<SimpleSelector>? unify(List<SimpleSelector> compound) {
-    if (compound.length == 1 && compound.first is UniversalSelector) {
-      return compound.first.unify([this]);
+    if (name == 'host' || name == 'host-context') {
+      if (!compound.every((simple) =>
+          simple is PseudoSelector &&
+          (simple.isHost || simple.selector != null))) {
+        return null;
+      }
+    } else if (compound.length == 1) {
+      var other = compound.first;
+      if (other is UniversalSelector ||
+          (other is PseudoSelector && (other.isHost || other.isHostContext))) {
+        return other.unify([this]);
+      }
     }
+
     if (compound.contains(this)) return compound;
 
     var result = <SimpleSelector>[];

lib/src/ast/selector/simple.dart

@@ -58,8 +58,12 @@ abstract class SimpleSelector extends Selector {
   /// Returns `null` if unification is impossible—for example, if there are
   /// multiple ID selectors.
   List<SimpleSelector>? unify(List<SimpleSelector> compound) {
-    if (compound.length == 1 && compound.first is UniversalSelector) {
-      return compound.first.unify([this]);
+    if (compound.length == 1) {
+      var other = compound.first;
+      if (other is UniversalSelector ||
+          (other is PseudoSelector && (other.isHost || other.isHostContext))) {
+        return other.unify([this]);
+      }
     }
     if (compound.contains(this)) return compound;
 

lib/src/ast/selector/universal.dart

@@ -24,10 +24,15 @@ class UniversalSelector extends SimpleSelector {
       visitor.visitUniversalSelector(this);
 
   List<SimpleSelector>? unify(List<SimpleSelector> compound) {
-    if (compound.first is UniversalSelector || compound.first is TypeSelector) {
-      var unified = unifyUniversalAndElement(this, compound.first);
+    var first = compound.first;
+    if (first is UniversalSelector || first is TypeSelector) {
+      var unified = unifyUniversalAndElement(this, first);
       if (unified == null) return null;
       return [unified, ...compound.skip(1)];
+    } else if (compound.length == 1 &&
+        first is PseudoSelector &&
+        (first.isHost || first.isHostContext)) {
+      return null;
     }
 
     if (namespace != null && namespace != "*") return [this, ...compound];

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.40.1
+version: 1.40.2-dev
 description: A Sass implementation in Dart.
 homepage: https://github.com/sass/dart-sass