The compiler automatically generates an IsReadOnlyAttribute on downlevel frameworks even if we have already defined it.

[teo-tsirpanis]

Please provide a succinct description of the issue.

Repro steps

Source:

#if NETSTANDARD2_0
namespace System.Runtime.CompilerServices

type IsReadOnlyAttribute() =
    inherit System.Attribute()
#endif

namespace fsharp_ns2_repro

[<Struct;System.Runtime.CompilerServices.IsReadOnly>]
type MyRecord = {X : int}

Project:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>net7.0;netstandard2.0</TargetFrameworks>
  </PropertyGroup>
  <ItemGroup>
    <Compile Include="Library.fs" />
  </ItemGroup>
</Project>

Expected behavior

Project compiles successfully on both frameworks.

Actual behavior

Project compiles successfully on .NET 7, fails to compile on .NET Standard 2.0 with:

FSC : error FS2014: A problem occurred writing the binary 'C:\Users\teo\code\fsharp-ns2-repro\obj\Debug\netstandard2.0\fsharp-ns2-repro.dll': duplicate entry 'System.Runtime.CompilerServices.IsReadOnlyAttribute' in type index table [C:\Users\teo
\code\fsharp-ns2-repro\fsharp-ns2-repro.fsproj::TargetFramework=netstandard2.0]

Known workarounds

Don't bother with IsReadOnlyAttribute and let the compiler apply it when it sees fit. I guessed that it would apply it to MyRecord, but it applies it only on X's getter. It's the same thing in this case, but I tried defining a method to the record, and was not marked as readonly.

Related information

Encountered when updating the .NET SDK from 7.0.2xx to 7.0.3xx (teo-tsirpanis/Farkle#104). Likely introduced in #14941.

[teo-tsirpanis]

That was fast, thanks! Will it be backported to 17.6?

[vzarytovskii]

That was fast, thanks! Will it be backported to 17.6?

We'll see what can be done, but likely it's not going to hit the bar.

Though still going to go to 7.0 (current target for 17.7 branch)