fix: mark :has selectors with multiple preceding selectors as used

Fixes #13717

There are two parts to this:

1. the parent selectors weren't passed along for the check inside `:has`, which in case of a leading combinator would mean it would always count as unused
2. In case if a selector like `x > y:has(z)`, the prior logic would correctly determine that for element `z` there's a match for the `:has` selector, by first checking its contents and then walking up the tree. But after it did that, it would try to walk up the tree once more, which is a) wasteful b) buggy because the tree walking mechanism would no longer be adjusted for the `:has` special case, resulting in false negatives. To fix that, the `:has` will return a new value from the function, signaling that it already fully checked the upper selectors, and so the function calling it will skip doing that.

Author: dummdidumm

.changeset/five-zoos-brush.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: mark `:has` selectors with multiple preceding selectors as used

packages/svelte/src/compiler/phases/2-analyze/css/css-prune.js

@@ -202,12 +202,17 @@ function apply_selector(relative_selectors, rule, element, stylesheet, check_has
 
 	const possible_match = relative_selector_might_apply_to_node(
 		relative_selector,
+		parent_selectors,
 		rule,
 		element,
 		stylesheet,
 		check_has
 	);
 
+	if (possible_match === 'definite_match') {
+		return true;
+	}
+
 	if (!possible_match) {
 		return false;
 	}
@@ -388,14 +393,16 @@ const regex_backslash_and_following_character = /\\(.)/g;
  * Ensure that `element` satisfies each simple selector in `relative_selector`
  *
  * @param {Compiler.Css.RelativeSelector} relative_selector
+ * @param {Compiler.Css.RelativeSelector[]} parent_selectors
  * @param {Compiler.Css.Rule} rule
  * @param {Compiler.AST.RegularElement | Compiler.AST.SvelteElement} element
  * @param {Compiler.Css.StyleSheet} stylesheet
  * @param {boolean} check_has Whether or not to check the `:has(...)` selectors
- * @returns {boolean}
+ * @returns {boolean | 'definite_match'}
  */
 function relative_selector_might_apply_to_node(
 	relative_selector,
+	parent_selectors,
 	rule,
 	element,
 	stylesheet,
@@ -446,7 +453,7 @@ function relative_selector_might_apply_to_node(
 						apply_combinator(
 							left_most_combinator,
 							selectors[0] ?? [],
-							[relative_selector],
+							[...parent_selectors, relative_selector],
 							rule,
 							element,
 							stylesheet,
@@ -478,7 +485,8 @@ function relative_selector_might_apply_to_node(
 			}
 		}
 
-		return true;
+		// We return this to signal the parent "don't bother checking the rest of the selectors, I already did that"
+		return 'definite_match';
 	}
 
 	for (const selector of other_selectors) {

packages/svelte/tests/css/samples/has/expected.css

@@ -85,3 +85,6 @@
 	/* (unused) x:has(> z) {
 		color: red;
 	}*/
+	x.svelte-xyz > y:where(.svelte-xyz):has(z:where(.svelte-xyz)) {
+		color: green;
+	}

packages/svelte/tests/css/samples/has/input.svelte

@@ -91,4 +91,7 @@
 	x:has(> z) {
 		color: red;
 	}
+	x > y:has(z) {
+		color: green;
+	}
 </style>