Error when using BeOfCase on a struct discriminated union with shared fields

[Numpsy]

Hi,

I was having a go at making some struct discriminated unions use shared/unnamed fields as described at https://learn.microsoft.com/en-us/dotnet/fsharp/whats-new/fsharp-9#field-sharing-for-struct-discriminated-unions in .NET 9 and hit an error in my unit tests.

Say I have a discriminated union like

    [<Struct>]
    type MyStructDu =
        | Field1 of string
        | Field2 of string

If I have a unit test on the first case like

    [<Fact>]
    let ``Struct DU case one`` () =
        let f1 = Field1 "F1"
        f1.Should().BeOfCase(Field1)

then it works ok, but for the second case, e.g

    [<Fact>]
    let ``Struct DU case two`` () =
        let f2 = Field2 "F2"
        f2.Should().BeOfCase(Field2)

Then i get

System.ArgumentException : This is not a valid tuple type for the F# reflection library. (Parameter 'types')

FSharpType.MakeTupleType(Type[] types) line 1148
[email protected](UnionCaseInfo unionCaseInfo) line 61
[email protected](a delegateArg0)
ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
[email protected](a a) line 32
MakeCaseTupleTypeCached.Static(UnionCaseInfo unionCaseInfo) line 79
[email protected](FSharpExpr expr) line 41
UnionAssertions.BeOfCase[a,b](Testable`1 t, FSharpExpr`1 caseConstructor, FSharpOption`1 because) line 47
BeOfCase.Struct DU case two() line 36

[cmeeren]

Thanks for the report. Seems to be an upstream issue. I created dotnet/fsharp#18121.

[Numpsy]

Thanks

[cmeeren]

Workaround here: dotnet/fsharp#18121 (comment)

Closing as it's an upstream issue.