Exception when comparing generated map containing private DU

I ran into an exception when running following test:

Message: System.Reflection.TargetInvocationException : Exception has been thrown by the target of an invocation. ---- System.ArgumentException : The type ‘SiteAgent.MessageGateway.Server.Tests.Foo+Bar’ is an F# union type but its representation is private. You must specify BindingFlags.NonPublic to access private type representations. Parameter name: unionType

open FsCheck
open FsCheck.Xunit

module Foo =
    type Bar = private Bar of string
    let create bar = Bar bar

module DataTests =
    [<Property>]
    let ``empty = generated nonEmpty`` (bars: Map<Foo.Bar, string>) =
        not (bars.IsEmpty) ==> lazy
        let emptyMap : Map<Foo.Bar, string> = Map.empty
        emptyMap = bars

When the map is created inside the test it fails correctly without the exception:

    [<Property>]
    let ``empty = nonEmpty``() =
        let bar = Foo.create "bar"
        let nonEmptyMap = Map [ bar, "bar" ]
        let emptyMap : Map<Foo.Bar, string> = Map.empty
        emptyMap = nonEmptyMap

map [] = map [(Bar “bar”, “bar”)] false Expected: True Actual: False

[kurtschelfthout]

Let’s say we’re considering whether to add it and if yes, how to add automatic generation/shrinking for private types. Imo we can add it but we should make it opt-in.

Meanwhile I’ll close this, as I don’t want to duplicate the discussion here. Feel free to add any thoughts to the other discussion, or re-open if there is anything unrelated that you still have a problem with.

[jbezak]

If I understand the pull request you’ve mentioned correctly it actually adds the functionality to generate private DU. That is also what I am seeing when debugging the tests.

I realized that the exception does not occur during the generation but during shrinking for failed tests. This test is passing but when it fails, shrinking will not help me to find the failing values.

  [<Property>]
  let ``generating nonEmpty`` (bars: Map<Foo.Bar, string>) =
      not (bars.IsEmpty) ==> lazy
      not bars.IsEmpty