C#: Handle async data flow in expression bodied callables

csharp/ql/src/semmle/code/csharp/Callable.qll

@@ -11,6 +11,7 @@ private import dotnet
 private import semmle.code.csharp.ExprOrStmtParent
 private import semmle.code.csharp.metrics.Complexity
 private import TypeRef
+private import semmle.code.csharp.frameworks.system.threading.Tasks
 
 /**
  * An element that can be called.
@@ -206,7 +207,11 @@ class Callable extends DotNet::Callable, Parameterizable, ExprOrStmtParent, @cal
     exists(ReturnStmt ret | ret.getEnclosingCallable() = this | e = ret.getExpr())
     or
     e = this.getExpressionBody() and
-    not this.getReturnType() instanceof VoidType
+    not this.getReturnType() instanceof VoidType and
+    (
+      not this.(Modifiable).isAsync() or
+      not this.getReturnType() instanceof SystemThreadingTasksTaskClass
+    )
   }
 
   /** Holds if this callable can yield return the expression `e`. */

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

@@ -788,7 +788,7 @@ private module Cached {
     or
     exists(Expr e |
       e = node1.asExpr() and
-      node2.(AsyncReturnNode).getReturnStmt().getExpr() = e and
+      node2.(AsyncReturnNode).getExpr() = e and
       c = getResultContent()
     )
     or
@@ -1411,23 +1411,23 @@ private module ReturnNodes {
    */
   class AsyncReturnNode extends ReturnNode, NodeImpl, TAsyncReturnNode {
     private ControlFlow::Nodes::ElementNode cfn;
-    private ReturnStmt rs;
+    private Expr expr;
 
-    AsyncReturnNode() { this = TAsyncReturnNode(cfn) and rs.getExpr().getAControlFlowNode() = cfn }
+    AsyncReturnNode() { this = TAsyncReturnNode(cfn) and expr = cfn.getElement() }
 
-    ReturnStmt getReturnStmt() { result = rs }
+    Expr getExpr() { result = expr }
 
     override NormalReturnKind getKind() { any() }
 
-    override Callable getEnclosingCallableImpl() { result = rs.getEnclosingCallable() }
+    override Callable getEnclosingCallableImpl() { result = expr.getEnclosingCallable() }
 
-    override Type getTypeImpl() { result = rs.getEnclosingCallable().getReturnType() }
+    override Type getTypeImpl() { result = expr.getEnclosingCallable().getReturnType() }
 
     override ControlFlow::Node getControlFlowNodeImpl() { result = cfn }
 
-    override Location getLocationImpl() { result = rs.getLocation() }
+    override Location getLocationImpl() { result = expr.getLocation() }
 
-    override string toStringImpl() { result = rs.toString() }
+    override string toStringImpl() { result = expr.toString() }
   }
 
   /**