in src/main/java/org/apache/maven/shared/dependency/analyzer/DefaultProjectDependencyAnalyzer.java [200:208]
private static Set<DependencyUsage> buildTestOnlyDependencyClasses(
Set<DependencyUsage> mainDependencyClasses, Set<DependencyUsage> testDependencyClasses) {
Set<DependencyUsage> testOnlyDependencyClasses = new HashSet<>(testDependencyClasses);
Set<String> mainDepClassNames = mainDependencyClasses.stream()
.map(DependencyUsage::getDependencyClass)
.collect(Collectors.toSet());
testOnlyDependencyClasses.removeIf(u -> mainDepClassNames.contains(u.getDependencyClass()));
return testOnlyDependencyClasses;
}