From 24cfd5a0ed5090f6e60971e4c2b676dca6d262fc Mon Sep 17 00:00:00 2001
From: Matthew Beck <me@mattrb.com>
Date: Fri, 7 Nov 2025 23:20:37 +0000
Subject: [PATCH] fix(compiler): support complex selectors in :nth-child()

:nth-child() (and its siblings) support complex expressions, e.g.
`:nth-child(2n of :is(.foo, .bar))`. Previously we'd choke because of
the `:is()`. Now, we reuse the `_parenSuffix` subexpression to match
nested parentheses the same way we do for :host() and :host-context().
Note that we only support 3 levels of nesting, so a selector like
`:nth-child(n of :is(:has(:not(.foo))))` will still break.

I'll say yet again that we really should add a proper parser so we stop
getting bug reports like this :)

Fixes #64913
---
 packages/compiler/src/shadow_css.ts                |  5 +++--
 .../test/shadow_css/host_and_host_context_spec.ts  |  3 +++
 .../compiler/test/shadow_css/shadow_css_spec.ts    | 14 ++++++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/packages/compiler/src/shadow_css.ts b/packages/compiler/src/shadow_css.ts
index a1d570ab292..c9d36d1e2c4 100644
--- a/packages/compiler/src/shadow_css.ts
+++ b/packages/compiler/src/shadow_css.ts
@@ -1029,9 +1029,9 @@ class SafeSelector {
 
     // Replaces the expression in `:nth-child(2n + 1)` with a placeholder.
     // WS and "+" would otherwise be interpreted as selector separators.
-    this._content = selector.replace(/(:nth-[-\w]+)(\([^)]+\))/g, (_, pseudo, exp) => {
+    this._content = selector.replace(nthRegex, (_, pseudo, exp) => {
       const replaceBy = `__ph-${this.index}__`;
-      this.placeholders.push(exp);
+      this.placeholders.push(`(${exp})`);
       this.index++;
       return pseudo + replaceBy;
     });
@@ -1076,6 +1076,7 @@ const _level1Parens = String.raw`(?:\(${_noParens}\)|${_noParens})+?`;
 // Matches content with at most TWO levels of nesting, e.g., "a(b(c)d)e"
 const _level2Parens = String.raw`(?:\(${_level1Parens}\)|${_noParens})+?`;
 const _parenSuffix = String.raw`(?:\((${_level2Parens})\))`;
+const nthRegex = new RegExp(String.raw`(:nth-[-\w]+)` + _parenSuffix, 'g');
 const _cssColonHostRe = new RegExp(_polyfillHost + _parenSuffix + '?([^,{]*)', 'gim');
 // note: :host-context patterns are terminated with `{`, as opposed to :host which
 // is both `{` and `,` because :host-context handles top-level commas differently.
diff --git a/packages/compiler/test/shadow_css/host_and_host_context_spec.ts b/packages/compiler/test/shadow_css/host_and_host_context_spec.ts
index 825f68b9f4a..db7c473d771 100644
--- a/packages/compiler/test/shadow_css/host_and_host_context_spec.ts
+++ b/packages/compiler/test/shadow_css/host_and_host_context_spec.ts
@@ -71,6 +71,9 @@ describe('ShadowCss, :host and :host-context', () => {
       expect(shim(':host:nth-child(8n+1) {}', 'contenta', 'a-host')).toEqualCss(
         '[a-host]:nth-child(8n+1) {}',
       );
+      expect(shim(':host(:nth-child(3n of :not(p, a))) {}', 'contenta', 'a-host')).toEqualCss(
+        '[a-host]:nth-child(3n of :not(p, a)) {}',
+      );
       expect(shim(':host:nth-of-type(8n+1) {}', 'contenta', 'a-host')).toEqualCss(
         '[a-host]:nth-of-type(8n+1) {}',
       );
diff --git a/packages/compiler/test/shadow_css/shadow_css_spec.ts b/packages/compiler/test/shadow_css/shadow_css_spec.ts
index 8705f65e5e6..2777b83272d 100644
--- a/packages/compiler/test/shadow_css/shadow_css_spec.ts
+++ b/packages/compiler/test/shadow_css/shadow_css_spec.ts
@@ -214,6 +214,20 @@ describe('ShadowCss', () => {
       'table[contenta] [contenta]:where(td, th):hover { color:lime;}',
     );
 
+    // :nth
+    expect(shim(':nth-child(3n of :not(p, a), :is(.foo)) {}', 'contenta', 'hosta')).toEqualCss(
+      '[contenta]:nth-child(3n of :not(p, a), :is(.foo)) {}',
+    );
+    expect(shim('li:nth-last-child(-n + 3) {}', 'contenta', 'a-host')).toEqualCss(
+      'li[contenta]:nth-last-child(-n + 3) {}',
+    );
+    expect(shim('dd:nth-last-of-type(3n) {}', 'contenta', 'a-host')).toEqualCss(
+      'dd[contenta]:nth-last-of-type(3n) {}',
+    );
+    expect(shim('dd:nth-of-type(even) {}', 'contenta', 'a-host')).toEqualCss(
+      'dd[contenta]:nth-of-type(even) {}',
+    );
+
     // complex selectors
     expect(shim(':host:is([foo],[foo-2])>div.example-2 {}', 'contenta', 'a-host')).toEqualCss(
       '[a-host]:is([foo],[foo-2]) > div.example-2[contenta] {}',