SONARJAVA-6095 S1166 should not report on exceptions using unnamed variable _ (#5436)

Author: tomasz-tylenda-sonarsource

java-checks-test-sources/default/src/main/java/checks/CatchUsesExceptionWithContextCheck.java

@@ -65,6 +65,9 @@ private void f(Exception x) {
         System.out.println("" + e);
       }
     }
+    try {
+    } catch (Exception _) {                     // Compliant
+    }
   }
 
   private void g() {

java-checks/src/main/java/org/sonar/java/checks/CatchUsesExceptionWithContextCheck.java

@@ -170,7 +170,7 @@ public void visitLambdaExpression(LambdaExpressionTree lambdaExpressionTree) {
 
   @Override
   public void visitCatch(CatchTree tree) {
-    if (!isExcludedType(tree.parameter().type()) && !excludedCatchTrees.contains(tree)) {
+    if (!isExcludedType(tree.parameter().type()) && !excludedCatchTrees.contains(tree) && !tree.parameter().simpleName().isUnnamedVariable()) {
       Symbol exception = tree.parameter().symbol();
       usageStatusStack.addFirst(new UsageStatus(exception.usages()));
       super.visitCatch(tree);

sonar-java-plugin/src/main/resources/org/sonar/l10n/java/rules/java/S1166.html

@@ -68,6 +68,16 @@ <h3>Exceptions</h3>
   LOGGER.warn(message); // Compliant - exception message logged with some contextual information
 }
 </pre>
+<p>Additionally, no issue will be raised if the exception parameter uses the unnamed variable <code>_</code>, which is available starting with Java
+21. The unnamed variable makes it explicit that the exception is caught but not needed in the handler (see {rule:java:S7467}).</p>
+<pre>
+try {
+  /* ... */
+} catch (Exception _) {
+  // Compliant - unnamed exception parameter is intentionally unused
+  return defaultValue;
+}
+</pre>
 <h2>Resources</h2>
 <ul>
   <li> OWASP - <a href="https://owasp.org/Top10/A09_2021-Security_Logging_and_Monitoring_Failures/">Top 10 2021 Category A9 - Security Logging and