Disabling default augmentation for struct DU breaks with .NET 8

[hyazinthh]

The compiler shipped with .NET 8.0.100 fails to compile code containing discriminated unions annotated with [<Struct>] and [<DefaultAugmentation(false)>] (error MethodDefNotFound) . Non-struct DUs do not have this problem.

Repro steps

module Test

[<Struct>]
[<DefaultAugmentation(false)>]
type Foo =
    | Baz of int
    | Bar

    member x.IsBaz =
        match x with | Baz _ -> true | _ -> false

    member x.IsBar =
        not x.IsBaz

[<EntryPoint>]
let main argv =
    let foo = Baz 42
    printfn "%A" foo
    0

test.zip

Expected behavior

Should compile.

Actual behavior

Fails with

error FS2014: A problem occurred writing the binary 'D:\test\obj\Debug\net6.0\test.dll': Error in pass3 for type Test, error: Error in GetMethodRefAsMethodDefIdx for mref = ("NewBaz", "Foo"), error: MethodDefNotFound

Related information

• Windows 10
• .NET 8.0.100 SDK

[T-Gro]

It is the following line causing the failure:

let foo = Baz 42

Compiler calls the auto-generated method to MakeBaz(42).
However, because of DefaultAugmentation(false), this maker method is not existing.

[T-Gro]

The solution probably will be to still emit the MakeBaz maker method in case of DefaultAugmentation(false), but keep it internal only, instead of public.

[T-Gro]

It will have to be public, since it can be also called from another project.
Only needed in the case of a non-nullary case (= having some fields).

[hyazinthh]

The build error no longer occurs with .NET 8.0.200 but I also noticed runtime failures when using types like above from libraries built with .NET 6.0.419:

System.MissingMethodException: Method not found: 'Library.Foo Library.Foo.NewBaz(Int32)'

test.zip

Steps to reproduce:

1. Build solution with .NET 8 (via VS 17.9 or change global.json accordingly)
2. Build library with .NET 6 (dotnet build .\library\library.fsproj)
3. .\bin\Debug\net6.0\main.exe

[T-Gro]

These seem to be the essential conditions:

• DU is a struct DU &&
• It has default augmentation (false) &&
• Has at least 1 case with data &&
• Is compiled with SDK 7 or lower in a library, and then consumed from code that uses SDK 8 or higher.

In the compiler used in older SDK (7 or lower), there used to be an (undocumented?) scenario with DefaultAugmentation(false) where compiled turned internal constructors public in order to create values of the cases.

https://sharplab.io/#v2:EYLgtghglgdgNAGxAMwM4B8DaAeAygFwCcBXAY3wD4BdAWACgcARAU2QmIXwEFiBzMZjHwR8UAPYwAFGwSpmASgDcAOTEBRAI7EICKPgCeKsQGExYAA4RCUVBOr0D55gAIAYmLHOAvPWd/n6M4AQhAAXs5iyM6w+L7+gSGEQA===

And this detail was then used across assembly boundary, assuming the implementation specifics of the (normally internal) constructors are same across assemblies.

Thanks for the repro @hyazinthh .
Can you also give an example of a real-world scenario where this is exhibited (e.g. a nuget consumed, which was built in SDK 6 and cannot be easily rebuilt?)

[hyazinthh]

This is where I ran into this issue: https://github.com/aardvark-platform/aardvark.media/blob/main/src/Aardvark.UI.Primitives/Animation/Core/Time.fs#L8

NuGet package: Aardvark.UI.Primitives

It's not that much of a problem, since we will probably move to .NET 8 anyway when .NET 6 goes out of support.