fix: allow `:global(..)` in compound selectors (#10266)

Someone could programmatically add a class to an element and Svelte doesn't see it, so having global be part of a modifier is necessary so that Svelte doesn't mark it as unused
fixes #10210

.changeset/serious-kids-deliver.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: allow `:global(..)` in compound selectors

packages/svelte/src/compiler/errors.js

@@ -100,7 +100,7 @@ const css = {
 		`:global(...) can be at the start or end of a selector sequence, but not in the middle`,
 	'invalid-css-global-selector': () => `:global(...) must contain exactly one selector`,
 	'invalid-css-global-selector-list': () =>
-		`:global(...) cannot be used to modify a selector, or be modified by another selector`,
+		`:global(...) must not contain type or universal selectors when used in a compound selector`,
 	'invalid-css-selector': () => `Invalid selector`,
 	'invalid-css-identifier': () => 'Expected a valid CSS identifier'
 };
@@ -447,19 +447,6 @@ const errors = {
 	// 	code: 'illegal-variable-declaration',
 	// 	message: 'Cannot declare same variable name which is imported inside <script context="module">'
 	// },
-	// css_invalid_global_selector: {
-	// 	code: 'css-invalid-global-selector',
-	// 	message: ':global(...) must contain a single selector'
-	// },
-	// css_invalid_global_selector_position: {
-	// 	code: 'css-invalid-global-selector-position',
-	// 	message:
-	// 		':global(...) not at the start of a selector sequence should not contain type or universal selectors'
-	// },
-	// css_invalid_selector: /** @param {string} selector */ (selector) => ({
-	// 	code: 'css-invalid-selector',
-	// 	message: `Invalid selector "${selector}"`
-	// }),
 	// invalid_directive_value: {
 	// 	code: 'invalid-directive-value',
 	// 	message:

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

@@ -177,18 +177,15 @@ export default class Selector {
 
 	validate_global_compound_selector() {
 		for (const block of this.blocks) {
+			if (block.selectors.length === 1) continue;
+
 			for (let i = 0; i < block.selectors.length; i++) {
 				const selector = block.selectors[i];
-				if (
-					selector.type === 'PseudoClassSelector' &&
-					selector.name === 'global' &&
-					block.selectors.length !== 1 &&
-					(i === block.selectors.length - 1 ||
-						block.selectors
-							.slice(i + 1)
-							.some((s) => s.type !== 'PseudoElementSelector' && s.type !== 'PseudoClassSelector'))
-				) {
-					error(selector, 'invalid-css-global-selector-list');
+				if (selector.type === 'PseudoClassSelector' && selector.name === 'global') {
+					const child = selector.args?.children[0].children[0];
+					if (child?.type === 'TypeSelector' && !/[.:#]/.test(child.name[0])) {
+						error(selector, 'invalid-css-global-selector-list');
+					}
 				}
 			}
 		}

packages/svelte/tests/css/samples/global-with-class/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: []
+});

packages/svelte/tests/css/samples/global-with-class/expected.css

@@ -0,0 +1,9 @@
+	div.svelte-xyz.blue {
+		color: blue;
+	}
+	span.blue.x.svelte-xyz {
+		color: blue;
+	}
+	span.x.svelte-xyz.bg {
+		background: red;
+	}

packages/svelte/tests/css/samples/global-with-class/expected.html

@@ -0,0 +1,2 @@
+<div class="svelte-xyz">someone could programmatically add a class to this, so having global be part of a modifier is necessary</div>
+<span class="x svelte-xyz">-</span>

packages/svelte/tests/css/samples/global-with-class/input.svelte

@@ -0,0 +1,14 @@
+<div>someone could programmatically add a class to this, so having global be part of a modifier is necessary</div>
+<span class="x">-</span>
+
+<style>
+	div:global(.blue) {
+		color: blue;
+	}
+	span:global(.blue).x {
+		color: blue;
+	}
+	span.x:global(.bg) {
+		background: red;
+	}
+</style>

packages/svelte/tests/validator/samples/css-invalid-global-placement-4/errors.json

@@ -1,7 +1,7 @@
 [
 	{
 		"code": "invalid-css-global-selector-list",
-		"message": ":global(...) cannot be used to modify a selector, or be modified by another selector",
+		"message": ":global(...) must not contain type or universal selectors when used in a compound selector",
 		"start": {
 			"line": 2,
 			"column": 5

packages/svelte/tests/validator/samples/css-invalid-global-placement-5/errors.json

@@ -1,7 +1,7 @@
 [
 	{
 		"code": "invalid-css-global-selector-list",
-		"message": ":global(...) cannot be used to modify a selector, or be modified by another selector",
+		"message": ":global(...) must not contain type or universal selectors when used in a compound selector",
 		"start": {
 			"line": 2,
 			"column": 5

sites/svelte-5-preview/src/routes/docs/content/03-appendix/02-breaking-changes.md

@@ -76,7 +76,7 @@ Assignments to destructured parts of a `@const` declaration are no longer allowe
 
 ### Stricter CSS `:global` selector validation
 
-Previously, a selector like `.foo :global(bar).baz` was valid. In Svelte 5, this is a validation error instead. The reason is that in this selector the resulting CSS would be equivalent to one without `:global` - in other words, `:global` is ignored in this case.
+Previously, a compound selector starting with a global modifier which has universal or type selectors (like `:global(span).foo`) was valid. In Svelte 5, this is a validation error instead. The reason is that in this selector the resulting CSS would be equivalent to one without `:global` - in other words, `:global` is ignored in this case.
 
 ### CSS hash position no longer deterministic