C#: Fix FP in expressions of the form (a?b:null)??d, where the null has a single successor, but the d is a join node.

Author: calumgrant

csharp/ql/src/Bad Practices/Control-Flow/ConstantCondition.ql

@@ -71,8 +71,10 @@ class ConstantNullnessCondition extends ConstantCondition {
 
   ConstantNullnessCondition() {
     forex(ControlFlow::Node cfn | cfn = this.getAControlFlowNode() |
-      exists(ControlFlow::SuccessorTypes::NullnessSuccessor t | exists(cfn.getASuccessorByType(t)) |
-        b = t.getValue()
+      exists(ControlFlow::SuccessorTypes::NullnessSuccessor t, ControlFlow::Node s |
+        s = cfn.getASuccessorByType(t) |
+        b = t.getValue() and
+        not s.isJoin()
       ) and
       strictcount(ControlFlow::SuccessorType t | exists(cfn.getASuccessorByType(t))) = 1
     )

csharp/ql/test/query-tests/Bad Practices/Control-Flow/ConstantCondition/ConstantCondition.cs

@@ -50,7 +50,7 @@ void M2(int i)
         j = (int?)i ?? 1; // BAD
         s = ""?.CommaJoinWith(s); // BAD
         s = s ?? ""; // GOOD
-        s = (i==0 ? s : null) ?? s;  // BAD (False positive)
+        s = (i==0 ? s : null) ?? s;
     }
 }
 

csharp/ql/test/query-tests/Bad Practices/Control-Flow/ConstantCondition/ConstantCondition.expected

@@ -4,7 +4,6 @@
 | ConstantCondition.cs:49:17:49:18 | "" | Expression is never 'null'. |
 | ConstantCondition.cs:50:13:50:19 | (...) ... | Expression is never 'null'. |
 | ConstantCondition.cs:51:13:51:14 | "" | Expression is never 'null'. |
-| ConstantCondition.cs:53:25:53:28 | null | Expression is always 'null'. |
 | ConstantCondition.cs:63:18:63:18 | 2 | Pattern never matches. |
 | ConstantCondition.cs:65:18:65:18 | 3 | Pattern always matches. |
 | ConstantCondition.cs:76:18:76:20 | access to type Int32 | Pattern never matches. |