in src/main/java/org/apache/maven/resolver/internal/ant/types/Exclusion.java [47:56]
public void validate(Task task) {
if (isReference()) {
getRef().validate(task);
} else {
if (groupId == null && artifactId == null && classifier == null && extension == null) {
throw new BuildException(
"You must specify at least one of " + "'groupId', 'artifactId', 'classifier' or 'extension'");
}
}
}