SONARJAVA-2944: reduce scope of the rule to avoid FP

java-checks-test-sources/src/main/java/checks/UnreachableCatchCheck.java

@@ -68,7 +68,7 @@ void unreachable(boolean cond) {
       // ...
     } catch (IOException e) { // Compliant
       // ...
-    } catch (CustomException e) { // Noncompliant [[sc=14;ec=29;secondary=67]]
+    } catch (CustomException e) { // FN, line 67 hide this catch, but we don't support correctly the presence of another type of Exception
       // ...
     }
 
@@ -77,7 +77,7 @@ void unreachable(boolean cond) {
       throwIOException();
     } catch (CustomDerivedException | IOException e) {
       // ...
-    } catch (CustomException e) { // Noncompliant
+    } catch (CustomException e) { // FN, we don't support correctly the presence of another type of Exception
       // ...
     }
 
@@ -215,6 +215,23 @@ public void close() throws FileNotFoundException {
       // ...
     }
 
+    try {
+      throwCustomDerivedException();
+      throwOtherCustomDerivedException();
+    } catch (CustomDerivedException e) {
+      // ...
+    } catch (CustomException e) { // Compliant
+      // ...
+    }
+
+    try {
+      throwBoth();
+    } catch (CustomDerivedException e) {
+      // ...
+    } catch (CustomException e) { // Compliant
+      // ...
+    }
+
   }
 
   void throwCustomException() throws CustomException {
@@ -225,6 +242,13 @@ void throwCustomDerivedException() throws CustomDerivedException {
     throw new CustomDerivedException();
   }
 
+  void throwBoth() throws CustomDerivedException,OtherCustomDerivedException {
+  }
+
+  void throwOtherCustomDerivedException() throws OtherCustomDerivedException {
+    throw new OtherCustomDerivedException();
+  }
+
   void throwCustomDerivedDerivedException() throws CustomDerivedDerivedException {
     throw new CustomDerivedDerivedException();
   }
@@ -247,6 +271,9 @@ public static class CustomException extends Exception {
   public static class CustomDerivedException extends CustomException {
   }
 
+  public static class OtherCustomDerivedException extends CustomException {
+  }
+
   public static class CustomDerivedDerivedException extends CustomDerivedException {
   }
 

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

@@ -134,12 +134,12 @@ private static boolean isChecked(Type type) {
   }
 
   private static boolean isHiding(Type derivedType, List<Type> thrownTypes) {
-    return thrownTypes.stream().allMatch(thrownType ->
-      // Only throwing a subtype of the first caught exception, hiding the base one
-      thrownType.isSubtypeOf(derivedType) ||
-      // Or throwing an unrelated exception
-      (!thrownType.isUnknown() && !derivedType.isUnknown() && !derivedType.isSubtypeOf(thrownType))
-    );
+    // All thrown exceptions are caught by the subtype exception, hiding the base one
+    // This logic could be improved to remove FN, but it's not trivial and source of FP.
+    return thrownTypes.stream()
+      .allMatch(thrownType ->
+        thrownType.isSubtypeOf(derivedType)
+      );
   }
 
   private static class ThrownExceptionCollector extends BaseTreeVisitor {