I am currently working on a .net 4.0 project which uses async/await quite heavily. We also are using the old AsyncTargeting pack rather than the RTM because we do not have stats on how many clients currently have the .net 4.0 KB 2468871 patch which enables PCL support. These issues affect the RTM version as well.
Recently I have upgraded to nCrunch 2.2 beta and ReSharper 8.1 both which ship with the upgraded nUnit 2.6.3 runner. After I upgraded these tools I noticed tests which should have been failing were passing and passing tests were being reported as failing but showing stack traces from a different test..
Also I was getting different results in nCrunch, R# and nUnits console runner. Something was broken.
NOTE: Async and TPL support is not supported in nUnit 2.x, but will be officially supported in v3.x and that it was a coincidence that it worked in 2.6.2. My discussions about the issues are at here on the nunit discussion board.
As far as I can tell, there is no reason that 4.0 cannot be supported because to offer framework support does not need any new features OR the classes in the Async Targeting Pack or .NET 4.5. At a minimum tests returning
Task should be supported as TPL was introduced into the CLR for net40.
Recently I added async void and Task support to BDDfy which targets .NET 4.0, also xUnit 1.9.x supports Tasks in the current released version and has backported
async void support to the 1.9.x codebase from the 2.0 and will be released if there is a need to release another patch release before 2.0 is released.
.NET 4.5 is fully supported and works as expected with nUnit, it is only .net 4.0 which is affected