From f426cfafb7f6557d0ab16b4f31726704aac4147c Mon Sep 17 00:00:00 2001
From: Lazizbek Ergashev <lazerg2@gmail.com>
Date: Tue, 28 Jul 2026 08:07:34 +0500
Subject: [PATCH] Keep leading `+` glued in An+B notation (#19738)

---
 changelog_unreleased/css/19738.md             | 18 +++++++
 src/language-css/printer-postcss.js           | 27 ++++++++--
 .../__snapshots__/format.test.js.snap         | 51 +++++++++++++++++++
 tests/format/css/combinator/an-plus-b.css     | 19 +++++++
 4 files changed, 110 insertions(+), 5 deletions(-)
 create mode 100644 changelog_unreleased/css/19738.md
 create mode 100644 tests/format/css/combinator/an-plus-b.css

diff --git a/changelog_unreleased/css/19738.md b/changelog_unreleased/css/19738.md
new file mode 100644
index 000000000..3c4add519
--- /dev/null
+++ b/changelog_unreleased/css/19738.md
@@ -0,0 +1,18 @@
+#### Keep leading `+` glued in An+B notation (#19738 by @lazerg)
+
+The An+B grammar doesn't allow whitespace between a leading `+` and the `n`, so the extra space produced invalid output.
+
+<!-- prettier-ignore -->
+```css
+/* Input */
+a:nth-child(+3n+2) {
+}
+
+/* Prettier stable */
+a:nth-child(+ 3n + 2) {
+}
+
+/* Prettier main */
+a:nth-child(+3n + 2) {
+}
+```
diff --git a/src/language-css/printer-postcss.js b/src/language-css/printer-postcss.js
index e04ec746a..1c30081be 100644
--- a/src/language-css/printer-postcss.js
+++ b/src/language-css/printer-postcss.js
@@ -350,13 +350,30 @@ function genericPrint(path, options, print) {
         node.value === ">>>"
       ) {
         const parentNode = path.parent;
-        const leading =
+        const isFirstNode =
           parentNode.type === "selector-selector" &&
-          parentNode.nodes[0] === node
-            ? ""
-            : line;
+          parentNode.nodes[0] === node;
+        const leading = isFirstNode ? "" : line;
 
-        return [leading, node.value, path.isLast ? "" : " "];
+        // The leading `+` in An+B notation (e.g. `:nth-child(+2n)`) must stay
+        // glued to the following token, a space there is invalid per spec.
+        const grandparentNode = path.grandparent;
+        const isNthLeadingSign =
+          isFirstNode &&
+          node.value === "+" &&
+          grandparentNode?.type === "selector-pseudo" &&
+          [
+            ":nth-child",
+            ":nth-last-child",
+            ":nth-of-type",
+            ":nth-last-of-type",
+          ].includes(grandparentNode.value.toLowerCase());
+
+        return [
+          leading,
+          node.value,
+          path.isLast || isNthLeadingSign ? "" : " ",
+        ];
       }
 
       const leading = node.value.trimStart().startsWith("(") ? line : "";
diff --git a/tests/format/css/combinator/__snapshots__/format.test.js.snap b/tests/format/css/combinator/__snapshots__/format.test.js.snap
index ee1c44007..2071350a8 100644
--- a/tests/format/css/combinator/__snapshots__/format.test.js.snap
+++ b/tests/format/css/combinator/__snapshots__/format.test.js.snap
@@ -1,5 +1,56 @@
 // Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
 
+exports[`an-plus-b.css format 1`] = `
+====================================options=====================================
+parsers: ["css"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+:nth-child(+2n),
+:nth-child(+n),
+:nth-child(+3n+2),
+:nth-last-child(+3n+1),
+:nth-of-type(+2n),
+:nth-last-of-type(+n+3),
+:nth-child(2n+1),
+:nth-child(-n+2),
+:not(:nth-child(+2n)),
+:not(:nth-child(+n)),
+:not(:nth-child(+3n+2)),
+:not(:nth-last-child(+3n+1)),
+:not(:nth-of-type(+2n)),
+:not(:nth-last-of-type(+n+3)),
+:not(:nth-child(2n+1)),
+:not(:nth-child(-n+2)) {}
+
+:has(+ img),
+:has(+img) {}
+
+=====================================output=====================================
+:nth-child(+2n),
+:nth-child(+n),
+:nth-child(+3n + 2),
+:nth-last-child(+3n + 1),
+:nth-of-type(+2n),
+:nth-last-of-type(+n + 3),
+:nth-child(2n + 1),
+:nth-child(-n + 2),
+:not(:nth-child(+2n)),
+:not(:nth-child(+n)),
+:not(:nth-child(+3n + 2)),
+:not(:nth-last-child(+3n + 1)),
+:not(:nth-of-type(+2n)),
+:not(:nth-last-of-type(+n + 3)),
+:not(:nth-child(2n + 1)),
+:not(:nth-child(-n + 2)) {
+}
+
+:has(+ img),
+:has(+ img) {
+}
+
+================================================================================
+`;
+
 exports[`combinator.css format 1`] = `
 ====================================options=====================================
 parsers: ["css"]
diff --git a/tests/format/css/combinator/an-plus-b.css b/tests/format/css/combinator/an-plus-b.css
new file mode 100644
index 000000000..513b34a3a
--- /dev/null
+++ b/tests/format/css/combinator/an-plus-b.css
@@ -0,0 +1,19 @@
+:nth-child(+2n),
+:nth-child(+n),
+:nth-child(+3n+2),
+:nth-last-child(+3n+1),
+:nth-of-type(+2n),
+:nth-last-of-type(+n+3),
+:nth-child(2n+1),
+:nth-child(-n+2),
+:not(:nth-child(+2n)),
+:not(:nth-child(+n)),
+:not(:nth-child(+3n+2)),
+:not(:nth-last-child(+3n+1)),
+:not(:nth-of-type(+2n)),
+:not(:nth-last-of-type(+n+3)),
+:not(:nth-child(2n+1)),
+:not(:nth-child(-n+2)) {}
+
+:has(+ img),
+:has(+img) {}