Wikimedia Bugzilla is closed!

Attempting to create a namespace alias which links to NS_MAIN results in the entry being completely ignored. Aliases to other namespaces work just fine.

I was able to solve this problem by editing the function Title::secureAndSplit() as follows:

			$m = array();
			if ( preg_match( $prefixRegexp, $dbkey, $m ) ) {
				$p = $m[1];
-				if ( $ns = $wgContLang->getNsIndex( $p ) ) {
+				if (( $ns = $wgContLang->getNsIndex( $p ) ) !== false) {
					# Ordinary namespace
					$dbkey = $m[2];
					$this->mNamespace = $ns;

The function Language::getNsIndex() explicitly returns FALSE if the namespace name is not found, but since the above code does not check the type of the return value, a namespace ID of 0 (that is, NS_MAIN) is treated as nonexistent.

A quick glance at SVN (in mediawiki/trunk/phase3) suggests that this issue may still be present in the latest version.