Allow asynchronous tests to run on Task<'a>

That FsCheck can run asynchronous tests is one of its major advantages over competitors. It’d be nice, however, if it could run methods/actions that return Task<'a> instead of simply Task.

Particularly, with the new task computation expression, one can easily write an asynchronous (integration) test, but even if such a test body ends with an assertion, the expression has the type Task<unit>, rather than Task.

One can get around this issue by upcasting the expression with :> Task. If, however, one wishes to avoid adding an extra line of code for only that purpose, one runs into this issue.

If, currently, one tries to pass a Task<unit> to Prop.forAll if compiles fine, but fails at run time with this error message:

No instances of class FsCheck.Testable+ITestable`1[T] for type
System.Threading.Tasks.Task`1[Microsoft.FSharp.Core.Unit]

This is on FsCheck.Xunit 3.0.0-beta2.

Since Task<'a> derives from Task is seems to me that a fix should be a fairly straightforward.

If you agree that such a fix is desired, I may try my hand at it myself. (I’ve been away from contributing to FsCheck for years, now, so I hope I’m not biting off more than I can chew.)

[kurtschelfthout]

Released in 3.0.0-rc1.

Also merged fscheck3 branch to master.

[kurtschelfthout]

@bartelink yes, I think it’s time for that. I’ll probably just do 3.0.0 even. If the other plans ever materialize, there’ll just have to be another version.