C#: Add ?? as a local dataflow step.

Author: calumgrant

csharp/ql/src/API Abuse/NoDisposeCallOnLocalIDisposable.ql

@@ -81,8 +81,6 @@ private class Conf extends DataFlow::Configuration {
 
   override predicate isAdditionalFlowStep(DataFlow::Node node1, DataFlow::Node node2) {
     node2.asExpr() = any(LocalScopeDisposableCreation other | other.getAnArgument() = node1.asExpr())
-    or
-    exists(NullCoalescingExpr nce | node1.asExpr() = nce.getRightOperand() and node2.asExpr() = nce)
   }
 
   override predicate isBarrierOut(DataFlow::Node node) {

csharp/ql/src/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -86,6 +86,10 @@ module LocalFlow {
         scope = e2 and
         isSuccessor = true
         or
+        e1 = e2.(NullCoalescingExpr).getAnOperand() and
+        scope = e2 and
+        isSuccessor = false
+        or
         e1 = e2.(SuppressNullableWarningExpr).getExpr() and
         scope = e2 and
         isSuccessor = true

csharp/ql/test/library-tests/dataflow/local/DataFlow.expected

@@ -8,6 +8,8 @@
 | LocalDataFlow.cs:412:15:412:20 | access to local variable sink70 |
 | LocalDataFlow.cs:420:19:420:24 | access to local variable sink71 |
 | LocalDataFlow.cs:430:23:430:28 | access to local variable sink72 |
+| LocalDataFlow.cs:445:15:445:20 | access to local variable sink73 |
+| LocalDataFlow.cs:446:15:446:20 | access to local variable sink74 |
 | LocalDataFlow.cs:472:15:472:21 | access to parameter tainted |
 | SSA.cs:9:15:9:22 | access to local variable ssaSink0 |
 | SSA.cs:25:15:25:22 | access to local variable ssaSink1 |

csharp/ql/test/library-tests/dataflow/local/DataFlowStep.expected

@@ -568,11 +568,15 @@
 | LocalDataFlow.cs:435:17:435:24 | access to local variable nonSink0 | LocalDataFlow.cs:443:22:443:29 | access to local variable nonSink0 |
 | LocalDataFlow.cs:437:18:437:33 | SSA def(nonSink17) | LocalDataFlow.cs:438:23:438:31 | access to local variable nonSink17 |
 | LocalDataFlow.cs:443:13:443:38 | SSA def(sink73) | LocalDataFlow.cs:445:15:445:20 | access to local variable sink73 |
+| LocalDataFlow.cs:443:22:443:29 | access to local variable nonSink0 | LocalDataFlow.cs:443:22:443:38 | ... ?? ... |
 | LocalDataFlow.cs:443:22:443:29 | access to local variable nonSink0 | LocalDataFlow.cs:444:31:444:38 | access to local variable nonSink0 |
 | LocalDataFlow.cs:443:22:443:38 | ... ?? ... | LocalDataFlow.cs:443:13:443:38 | SSA def(sink73) |
+| LocalDataFlow.cs:443:34:443:38 | access to local variable sink0 | LocalDataFlow.cs:443:22:443:38 | ... ?? ... |
 | LocalDataFlow.cs:443:34:443:38 | access to local variable sink0 | LocalDataFlow.cs:444:22:444:26 | access to local variable sink0 |
 | LocalDataFlow.cs:444:13:444:38 | SSA def(sink74) | LocalDataFlow.cs:446:15:446:20 | access to local variable sink74 |
+| LocalDataFlow.cs:444:22:444:26 | access to local variable sink0 | LocalDataFlow.cs:444:22:444:38 | ... ?? ... |
 | LocalDataFlow.cs:444:22:444:38 | ... ?? ... | LocalDataFlow.cs:444:13:444:38 | SSA def(sink74) |
+| LocalDataFlow.cs:444:31:444:38 | access to local variable nonSink0 | LocalDataFlow.cs:444:22:444:38 | ... ?? ... |
 | LocalDataFlow.cs:464:28:464:30 | this | LocalDataFlow.cs:464:41:464:45 | this access |
 | LocalDataFlow.cs:464:50:464:52 | this | LocalDataFlow.cs:464:56:464:60 | this access |
 | LocalDataFlow.cs:464:50:464:52 | value | LocalDataFlow.cs:464:64:464:68 | access to parameter value |