SONARJAVA-6112 S8446 Fix NPE when accessing super with broken semantics (#5496)

Co-authored-by: Tomasz Tylenda <tomasz.tylenda@sonarsource.com>

Author: rombirli

java-checks-test-sources/default/src/main/files/non-compiling/checks/MultipleMainInstancesNonCompilingSample.java

@@ -0,0 +1,18 @@
+class MultipleMainInstancesSample {
+  public class UnknownSuper extends UndefinedParent {
+    void main() { // Compliant - cannot verify if UndefinedParent has a main method
+    }
+  }
+
+  // Decoy non-compliant example in the non-compiling test, so we can verifyIssues
+  public static class NonCompliant {
+    public static void main(String[] args) { // Noncompliant {{At most one main method should be defined in a class.}}
+//                     ^^^^
+      System.out.println("Static main detected; shadowing instance main.");
+    }
+
+    void main() {
+      System.out.println("Unreachable entry point due to static precedence.");
+    }
+  }
+}

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

@@ -82,6 +82,9 @@ private List<MethodTree> findMainMethodsInSuperclasses(ClassTree ct) {
     var superClass = ct.superClass();
     while (superClass != null) {
       var superClassTree = superClass.symbolType().symbol().declaration();
+      if (superClassTree == null) {
+        break;
+      }
       findMainMethodsInMembers(superClassTree)
         .forEach(mains::add);
       superClass = superClassTree.superClass();

java-checks/src/test/java/org/sonar/java/checks/MultipleMainInstancesCheckTest.java

@@ -16,26 +16,38 @@
  */
 package org.sonar.java.checks;
 
-import org.junit.jupiter.api.Test;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
 import org.sonar.java.checks.verifier.CheckVerifier;
 
 import static org.sonar.java.checks.verifier.TestUtils.mainCodeSourcesPath;
+import static org.sonar.java.checks.verifier.TestUtils.nonCompilingTestSourcesPath;
 
 class MultipleMainInstancesCheckTest {
 
-  @Test
-  void test() {
+  static Stream<String> testSamples() {
+    return Stream.of(
+      mainCodeSourcesPath("checks/MultipleMainInstancesSample.java"),
+      nonCompilingTestSourcesPath("checks/MultipleMainInstancesNonCompilingSample.java")
+    );
+  }
+
+  @ParameterizedTest
+  @MethodSource("testSamples")
+  void test(String file) {
     CheckVerifier.newVerifier()
-      .onFile(mainCodeSourcesPath("checks/MultipleMainInstancesSample.java"))
+      .onFile(file)
       .withCheck(new MultipleMainInstancesCheck())
       .withJavaVersion(25)
       .verifyIssues();
   }
 
-  @Test
-  void test_java_24() {
+  @ParameterizedTest
+  @MethodSource("testSamples")
+  void test_java_24(String file) {
     CheckVerifier.newVerifier()
-      .onFile(mainCodeSourcesPath("checks/MultipleMainInstancesSample.java"))
+      .onFile(file)
       .withCheck(new MultipleMainInstancesCheck())
       .withJavaVersion(24)
       .verifyNoIssues();