Merge pull request #14643 from jasonmalinowski/async-lazy-can-block-on-background-thread

Allow AsyncLazy.GetValue to be called on a background thread

src/Workspaces/Core/Portable/Utilities/AsyncLazy`1.cs

@@ -222,7 +222,11 @@ public override T GetValue(CancellationToken cancellationToken)
                     StartAsynchronousComputation(newAsynchronousComputation.Value, requestToCompleteSynchronously: request, callerCancellationToken: cancellationToken);
                 }
 
-                return request.Task.WaitAndGetResult(cancellationToken);
+                // The reason we have synchronous codepaths in AsyncLazy is to support the synchronous requests for syntax trees
+                // that we may get from the compiler. Thus, it's entirely possible that this will be requested by the compiler or
+                // an analyzer on the background thread when another part of the IDE is requesting the same tree asynchronously.
+                // In that case we block the synchronous request on the asynchronous request, since that's better than alternatives.
+                return request.Task.WaitAndGetResult_CanCallOnBackground(cancellationToken);
             }
             else
             {