sync call hangs when it follows an async call (asp.net core application)

[bkindersleytrulioo]

we recently converted an asp.net core application to act as a nettcp wcf client (it was proxying through a REST api before)

we encountered a problem when a sync call follows an async call: the sync call times out, despite finishing quickly on the server side.

i found a few (relatively old) posts online that describe a similar problem:
https://aloiskraus.wordpress.com/2018/07/01/why-does-my-synchronous-wcf-call-hang/
https://stackoverflow.com/questions/37087103/wcf-client-hangs-on-any-operation-after-awaiting-async-operation-in-console-appl

and it does seem to be related to the SynchronizationContext (or lack thereof)

i have a small repro in a console app (which also doesn't have a SynchronizationContext) here:
https://github.com/bkindersleytrulioo/WCFDeadlock

i would expect that the sync call should be able to complete as it does when it does not follow an async call

at this point, i don't know enough about SynchronizationContexts or the WCF internals to really understand the issue or know what is a reasonable fix, or even to tell if it could be at all related to:
#4862

i found a couple of "workarounds" (as documented in the repro), but none that seem like very good options for a real application; we would like to convert all the usages in the client to async and avoid the problem that way, but it's not a trivial application, and that's not something we can do all at once

[HongGit]

This was fixed in WCF Client 6.0 release.