Tests run in test explorer never ends

[vitaliipanchenko]

Steps to reproduce:
1 Create F# XUnit project template
2 run following test in test explorer

open FsCheck.Xunit

type Member () =
    member val Name = "" with get, set

[<Property>]
let ``test mutation`` (m :Member)  =
    m.Name <- "1"
    m.Name = "0"

OS Windows 10
IDE VS 16.9.4, Rider 2021.1.2
FsCheck.Xunit 2.15.2

[ploeh]

FWIW, I can reproduce in both Visual Studio Community 2019 16.9.4 and dotnet test.

[kurtschelfthout]

Yes, this is because m is captured for shrinking. So when the test fails, FsCheck tries to shrink m with Name set to "1". It tries to shrink to say "", the empty string. But to check if the shrink is succesful, i.e. if the test is still failing, it needs to run the test. Which then sets Name to "1" again, the test fails again, and FsCheck tries to shrink the "1" to "" again, ad infinitum.

Basically if you want to mutate an object inside a test, you'll need to copy it first.

[vitaliipanchenko]

As proposal maybe it is worth to add custom exception for the case.

[kurtschelfthout]

I don't really see how to detect this case in particular (at least not without adding a bunch of complexity).

I think the best thing to do is to limit the number of shrink steps to some maximum, so at least it doesn't hang. Can also add some explanation of this possible cause in case that limit is hit.

[kurtschelfthout]

Released in 2.15.3