Issue with consuming DLL when using LangVersion set to 8.0 with .NET SDK 9

[ljb]

Recently I changed a large F# code base to work with .NET SDK 9 and F# 9. To start with I didn't change LangVersion which gave me a lot of errors of the type:

    FSC : error FS0229: Error opening binary file 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll': Error reading/writing metadata for the F# compiled DLL 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll'. Was the DLL compiled with an earlier version of the F# compiler? (error: 'u_ty - 1/B, byte = 1').
    FSC : error FS3160: Problem reading assembly 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll': The exception has been reported. This internal exception should now be caught at an error recovery point on the stack. Original message: Error opening binary file 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll': Error reading/writing metadata for the F# compiled DLL 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll'. Was the DLL compiled with an earlier version of the F# compiler? (error: 'u_ty - 1/B, byte = 1').)

Repro steps
Use .NET SDK 9.

Have a project that has LangVersion set to 8.0:

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <LangVersion>8.0</LangVersion>
        <TargetFramework>net9.0</TargetFramework>
    </PropertyGroup>

    <ItemGroup>
        <Compile Include="Library.fs"/>
    </ItemGroup>

</Project>

Have the following code in Library.fs:

module MyModule

    let convert (lookup: 'a -> 'b) (id: 'a): unit =
        lookup id
        |> Option.ofObj
        |> ignore

Have a project that has a project reference to the project above. It doesn't matter what LangVersion or TargetFramework that project has.

A zip-file with this example is attached. Running dotnet build results in errors.
fsharp-error.zip

Expected behavior

It is possible to build the projects without errors.

Actual behavior

I get errors of the following type:

    FSC : error FS0229: Error opening binary file 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll': Error reading/writing metadata for the F# compiled DLL 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll'. Was the DLL compiled with an earlier version of the F# compiler? (error: 'u_ty - 1/B, byte = 1').
    FSC : error FS3160: Problem reading assembly 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll': The exception has been reported. This internal exception should now be caught at an error recovery point on the stack. Original message: Error opening binary file 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll': Error reading/writing metadata for the F# compiled DLL 'C:\repos\fsharp-test\ClassLibrary1\obj\Debug\net8.0\ref\ClassLibrary1.dll'. Was the DLL compiled with an earlier version of the F# compiler? (error: 'u_ty - 1/B, byte = 1').)

Known workarounds

Set LangVersion to 9.0 or use .NET SDK 8

Related information

A similar problem was discussed in #14313.

This issue is somewhat different I think. Here you get errors even if the same target framework is used for both projects. It is only LangVersion that cause the problem.

With .NET SDK 8.0.403 it works with LangVersion set both to 7.0 and 8.0.

I have a really large F# code base, and is only the kind of construct that I have in Library.fs that causes problems.

• Windows 11
• .NET SDK 9.0.100

[vzarytovskii]

Some feature which changes picked data, will be easy to pinpoint. Not sure how to mitigate though.