Keep leading + glued in An+B notation (#19738)

changelog_unreleased/css/19738.md

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+#### 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) {
+}
+```