pat.loc = "unknown location"
	} else {
		pat.loc = fmt.Sprintf("%s:%d", file, line)
	}

	mux.mu.Lock()
	defer mux.mu.Unlock()
	// Check for conflict.
	if err := mux.index.possiblyConflictingPatterns(pat, func(pat2 *pattern) error {
		if pat.conflictsWith(pat2) {
			d := describeConflict(pat, pat2)
			return fmt.Errorf("pattern %q (registered at %s) conflicts with pattern %q (registered at %s):\n%s",