C#: Introduce type for Synthetic fields.

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

@@ -759,7 +759,8 @@ private module Cached {
   newtype TContent =
     TFieldContent(Field f) { f.isUnboundDeclaration() } or
     TPropertyContent(Property p) { p.isUnboundDeclaration() } or
-    TElementContent()
+    TElementContent() or
+    TSyntheticFieldContent(SyntheticField f)
 
   pragma[nomagic]
   private predicate commonSubTypeGeneral(DataFlowTypeOrUnifiable t1, RelevantDataFlowType t2) {
@@ -2038,3 +2039,10 @@ predicate additionalLambdaFlowStep(Node nodeFrom, Node nodeTo, boolean preserves
 predicate allowParameterReturnInSelf(ParameterNode p) {
   FlowSummaryImpl::Private::summaryAllowParameterReturnInSelf(p)
 }
+
+abstract class SyntheticField extends string {
+  bindingset[this]
+  SyntheticField() { any() }
+
+  Type getType() { result instanceof ObjectType }
+}

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPublic.qll

@@ -224,6 +224,16 @@ class FieldContent extends Content, TFieldContent {
   deprecated override Gvn::GvnType getType() { result = Gvn::getGlobalValueNumber(f.getType()) }
 }
 
+class SyntheticFieldContent extends Content, TSyntheticFieldContent {
+  private SyntheticField f;
+
+  SyntheticFieldContent() { this = TSyntheticFieldContent(f) }
+
+  SyntheticField getField() { result = f }
+
+  override string toString() { result = "synthetic " + f.toString() }
+}
+
 /** A reference to a property. */
 class PropertyContent extends Content, TPropertyContent {
   private Property p;

csharp/ql/lib/semmle/code/csharp/dataflow/internal/FlowSummaryImplSpecific.qll

@@ -29,6 +29,8 @@ DataFlowType getContentType(Content c) {
     or
     t = c.(PropertyContent).getProperty().getType()
     or
+    t = c.(SyntheticFieldContent).getField().getType()
+    or
     c instanceof ElementContent and
     t instanceof ObjectType // we don't know what the actual element type is
   )