CPP: Fix UncheckedLeapYearAfterYearModification FPs.

cpp/ql/src/Likely Bugs/Leap Year/LeapYear.qll

@@ -91,6 +91,9 @@ abstract class LeapYearFieldAccess extends YearFieldAccess {
             mulBy100.getAChild().getValue().toInt() = 100 )))
         or exists( BinaryArithmeticOperation bao |
           bao = op.getAnOperand()
+          // we're specifically interested in calculations that update the existing
+          // value (like `x = x + 1`), so look for a child `YearFieldAccess`.
+          and bao.getAChild*() instanceof YearFieldAccess
           and not ( bao.getAChild().getValue().toInt() = 1900
             or bao.getAChild().getValue().toInt() = 2000
             or bao.getAChild().getValue().toInt() = 1980 

cpp/ql/test/query-tests/Likely Bugs/Leap Year/UncheckedLeapYearAfterYearModification/UncheckedLeapYearAfterYearModification.expected

@@ -9,4 +9,3 @@
 | test.cpp:636:11:636:17 | tm_year | Field $@ on variable $@ has been modified, but no appropriate check for LeapYear was found. | test.cpp:56:6:56:12 | tm_year | tm_year | test.cpp:628:12:628:19 | timeinfo | timeinfo |
 | test.cpp:640:5:640:9 | wYear | Field $@ on variable $@ has been modified, but no appropriate check for LeapYear was found. | test.cpp:12:7:12:11 | wYear | wYear | test.cpp:629:13:629:14 | st | st |
 | test.cpp:642:5:642:9 | wYear | Field $@ on variable $@ has been modified, but no appropriate check for LeapYear was found. | test.cpp:12:7:12:11 | wYear | wYear | test.cpp:629:13:629:14 | st | st |
-| test.cpp:677:5:677:11 | tm_year | Field $@ on variable $@ has been modified, but no appropriate check for LeapYear was found. | test.cpp:56:6:56:12 | tm_year | tm_year | test.cpp:664:12:664:12 | t | t |