C#: Fix for false positive.

Author: calumgrant

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

@@ -54,7 +54,16 @@ private class Conf extends DataFlow::Configuration {
       )
       or
       // A disposing method
-      exists(Call c, Parameter p | e = c.getArgumentForParameter(p) | mayBeDisposed(p))
+      exists(Call c, Parameter p |
+        e = c.getArgumentForParameter(p)
+        or
+        // e.g `Stream.Create(input ?? new TextReader())`
+        exists(NullCoalescingExpr nce |
+          nce = c.getArgumentForParameter(p) and e = nce.getRightOperand()
+        )
+      |
+        mayBeDisposed(p)
+      )
       or
       // Things that are assigned to fields, properties, or indexers may be disposed
       exists(AssignableDefinition def, Assignable a |

csharp/ql/test/query-tests/API Abuse/NoDisposeCallOnLocalIDisposable/NoDisposeCallOnLocalIDisposable.expected

@@ -2,5 +2,4 @@
 | NoDisposeCallOnLocalIDisposable.cs:53:18:53:73 | object creation of type FileStream | Disposable 'FileStream' is created here but is not disposed. |
 | NoDisposeCallOnLocalIDisposable.cs:54:9:54:64 | object creation of type FileStream | Disposable 'FileStream' is created here but is not disposed. |
 | NoDisposeCallOnLocalIDisposable.cs:76:25:76:71 | call to method Create | Disposable 'XmlReader' is created here but is not disposed. |
-| NoDisposeCallOnLocalIDisposable.cs:88:42:88:64 | object creation of type StringReader | Disposable 'StringReader' is created here but is not disposed. |
 | NoDisposeCallOnLocalIDisposableBad.cs:8:22:8:56 | object creation of type FileStream | Disposable 'FileStream' is created here but is not disposed. |