fix(compiler): support multiple selectors in `:host-context()` (#40494)

The previous commits refactored the `ShadowCss` emulator to support
desirable use-cases of `:host-context()`, but it dropped support
for passing a comma separated list of selectors to the `:host-context()` .

This commit rectifies that omission, despite the use-case not being
valid according to the ShadowDOM spec, to ensure backward compatibility
with the previous implementation.

PR Close #40494

packages/compiler/src/shadow_css.ts

@@ -295,26 +295,62 @@ export class ShadowCss {
   private _convertColonHostContext(cssText: string): string {
     return cssText.replace(_cssColonHostContextReGlobal, selectorText => {
       // We have captured a selector that contains a `:host-context` rule.
-      // There may be more than one so `selectorText` could look like:
+
+      // For backward compatibility `:host-context` may contain a comma separated list of selectors.
+      // Each context selector group will contain a list of host-context selectors that must match
+      // an ancestor of the host.
+      // (Normally `contextSelectorGroups` will only contain a single array of context selectors.)
+      const contextSelectorGroups: string[][] = [[]];
+
+      // There may be more than `:host-context` in this selector so `selectorText` could look like:
       // `:host-context(.one):host-context(.two)`.
-
-      const contextSelectors: string[] = [];
-      let match: RegExpMatchArray|null;
-
       // Execute `_cssColonHostContextRe` over and over until we have extracted all the
       // `:host-context` selectors from this selector.
+      let match: RegExpMatchArray|null;
       while (match = _cssColonHostContextRe.exec(selectorText)) {
         // `match` = [':host-context(<selectors>)<rest>', <selectors>, <rest>]
-        const contextSelector = (match[1] ?? '').trim();
+
-        if (contextSelector !== '') {
+        // The `<selectors>` could actually be a comma separated list: `:host-context(.one, .two)`.
-          contextSelectors.push(contextSelector);
+        const newContextSelectors =
+            (match[1] ?? '').trim().split(',').map(m => m.trim()).filter(m => m !== '');
+
+        // We must duplicate the current selector group for each of these new selectors.
+        // For example if the current groups are:
+        // ```
+        // [
+        //   ['a', 'b', 'c'],
+        //   ['x', 'y', 'z'],
+        // ]
+        // ```
+        // And we have a new set of comma separated selectors: `:host-context(m,n)` then the new
+        // groups are:
+        // ```
+        // [
+        //   ['a', 'b', 'c', 'm'],
+        //   ['x', 'y', 'z', 'm'],
+        //   ['a', 'b', 'c', 'n'],
+        //   ['x', 'y', 'z', 'n'],
+        // ]
+        // ```
+        const contextSelectorGroupsLength = contextSelectorGroups.length;
+        repeatGroups(contextSelectorGroups, newContextSelectors.length);
+        for (let i = 0; i < newContextSelectors.length; i++) {
+          for (let j = 0; j < contextSelectorGroupsLength; j++) {
+            contextSelectorGroups[j + (i * contextSelectorGroupsLength)].push(
+                newContextSelectors[i]);
+          }
         }
+
+        // Update the `selectorText` and see repeat to see if there are more `:host-context`s.
         selectorText = match[2];
       }
 
       // The context selectors now must be combined with each other to capture all the possible
-      // selectors that `:host-context` can match.
+      // selectors that `:host-context` can match. See `combineHostContextSelectors()` for more
-      return combineHostContextSelectors(contextSelectors, selectorText);
+      // info about how this is done.
+      return contextSelectorGroups
+          .map(contextSelectors => combineHostContextSelectors(contextSelectors, selectorText))
+          .join(', ');
     });
   }
 
@@ -714,3 +750,23 @@ function combineHostContextSelectors(contextSelectors: string[], otherSelectors:
               `${s}${hostMarker}${otherSelectors}, ${s} ${hostMarker}${otherSelectors}`)
       .join(',');
 }
+
+/**
+ * Mutate the given `groups` array so that there are `multiples` clones of the original array
+ * stored.
+ *
+ * For example `repeatGroups([a, b], 3)` will result in `[a, b, a, b, a, b]` - but importantly the
+ * newly added groups will be clones of the original.
+ *
+ * @param groups An array of groups of strings that will be repeated. This array is mutated
+ *     in-place.
+ * @param multiples The number of times the current groups should appear.
+ */
+export function repeatGroups<T>(groups: string[][], multiples: number): void {
+  const length = groups.length;
+  for (let i = 1; i < multiples; i++) {
+    for (let j = 0; j < length; j++) {
+      groups[j + (i * length)] = groups[j].slice(0);
+    }
+  }
+}

packages/compiler/test/shadow_css_spec.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {CssRule, processRules, ShadowCss} from '@angular/compiler/src/shadow_css';
+import {CssRule, processRules, repeatGroups, ShadowCss} from '@angular/compiler/src/shadow_css';
 import {normalizeCSS} from '@angular/platform-browser/testing/src/browser_util';
 
 {
@@ -249,14 +249,27 @@ import {normalizeCSS} from '@angular/platform-browser/testing/src/browser_util';
             ]);
       });
 
-      // This test is checking backward compatibility.
+      // It is not clear what the behavior should be for a `:host-context` with no selectors.
-      // It is not clear what the behaviour should be for a `:host-context` with no selectors.
+      // This test is checking that the result is backward compatible with previous behavior.
-      // Arguably it is actually an error that should be reported.
+      // Arguably it should actually be an error that should be reported.
       it('should handle :host-context with no ancestor selectors', () => {
-        expect(s('.outer :host-context .inner {}', 'contenta', 'a-host'))
+        expect(s(':host-context .inner {}', 'contenta', 'a-host'))
-            .toEqual('.outer [a-host] .inner[contenta] {}');
+            .toEqual('[a-host] .inner[contenta] {}');
-        expect(s('.outer :host-context() .inner {}', 'contenta', 'a-host'))
+        expect(s(':host-context() .inner {}', 'contenta', 'a-host'))
-            .toEqual('.outer [a-host] .inner[contenta] {}');
+            .toEqual('[a-host] .inner[contenta] {}');
+      });
+
+      // More than one selector such as this is not valid as part of the :host-context spec.
+      // This test is checking that the result is backward compatible with previous behavior.
+      // Arguably it should actually be an error that should be reported.
+      it('should handle selectors', () => {
+        expect(s(':host-context(.one,.two) .inner {}', 'contenta', 'a-host'))
+            .toEqual(
+                '.one[a-host] .inner[contenta], ' +
+                '.one [a-host] .inner[contenta], ' +
+                '.two[a-host] .inner[contenta], ' +
+                '.two [a-host] .inner[contenta] ' +
+                '{}');
       });
     });
 
@@ -482,4 +495,32 @@ import {normalizeCSS} from '@angular/platform-browser/testing/src/browser_util';
       });
     });
   });
+
+  describe('repeatGroups()', () => {
+    it('should do nothing if `multiples` is 0', () => {
+      const groups = [['a1', 'b1', 'c1'], ['a2', 'b2', 'c2']];
+      repeatGroups(groups, 0);
+      expect(groups).toEqual([['a1', 'b1', 'c1'], ['a2', 'b2', 'c2']]);
+    });
+
+    it('should do nothing if `multiples` is 1', () => {
+      const groups = [['a1', 'b1', 'c1'], ['a2', 'b2', 'c2']];
+      repeatGroups(groups, 1);
+      expect(groups).toEqual([['a1', 'b1', 'c1'], ['a2', 'b2', 'c2']]);
+    });
+
+    it('should add clones of the original groups if `multiples` is greater than 1', () => {
+      const group1 = ['a1', 'b1', 'c1'];
+      const group2 = ['a2', 'b2', 'c2'];
+      const groups = [group1, group2];
+      repeatGroups(groups, 3);
+      expect(groups).toEqual([group1, group2, group1, group2, group1, group2]);
+      expect(groups[0]).toBe(group1);
+      expect(groups[1]).toBe(group2);
+      expect(groups[2]).not.toBe(group1);
+      expect(groups[3]).not.toBe(group2);
+      expect(groups[4]).not.toBe(group1);
+      expect(groups[5]).not.toBe(group2);
+    });
+  });
 }