Interfaces don't forward static members to other interfaces

[Shadowblitz16]

Description

Interfaces don't forward static members to other interfaces

for example this doesn't work...

public interface ICreatable<out T> where T : ICreatable<T>
{
    static abstract T Create();
}
public interface ICreatable<out T, in TArgs> where T : ICreatable<T, TArgs>
{
    static abstract T Create(TArgs args);
}

public interface IWindow : ICreatable<IWindow, WindowArgs>
{ 
    uint Width  { get; }
    uint Height { get; }
    
}

It complains when I pass IWindow to ICreatable

The interface 'HazelSharp.Common.Windowing.IWindow' cannot be used as type parameter 'T' in the generic interface 'HazelSharp.Common.Patterns.ICreatable<out T,in TArgs>'. The constraint interface 'HazelSharp.Common.Patterns.ICreatable<HazelSharp.Common.Windowing.IWindow,HazelSharp.Common.Windowing.WindowArgs>' or its base interface has static abstract members.

Reproduction Steps

• Copy the code
• Paste it in your IDE (I'm using rider)
• See error

Expected behavior

It would forward the static member to the IWindow interface so that any class or struct that eventually implements IWindow has to implement ICreatable too

Actual behavior

It errors...

The interface 'HazelSharp.Common.Windowing.IWindow' cannot be used as type parameter 'T' in the generic interface 'HazelSharp.Common.Patterns.ICreatable<out T,in TArgs>'. The constraint interface 'HazelSharp.Common.Patterns.ICreatable<HazelSharp.Common.Windowing.IWindow,HazelSharp.Common.Windowing.WindowArgs>' or its base interface has static abstract members.

Regression?

Idk

Known Workarounds

None that I know of.

Configuration

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

    <PropertyGroup>
        <TargetFramework>net6.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <LangVersion>11</LangVersion>
        <EnablePreviewFeatures>true</EnablePreviewFeatures>
        <WarningsAsErrors>Nullable</WarningsAsErrors>
    </PropertyGroup>

    <ItemGroup>
      <PackageReference Include="OpenTK" Version="4.7.4" />
    </ItemGroup>

    <ItemGroup>
      <Folder Include="Platform" />
    </ItemGroup>

</Project>

Other information

No response

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[jaredpar]

Thanks for taking the time to file this issue. Unfortunately it's hard to determine exactly what the problem is here because the sample is incomplete. Could you provide a complete sample, including WindowArgs, to help us see what the issue is?

[Shadowblitz16]

Thanks for taking the time to file this issue. Unfortunately it's hard to determine exactly what the problem is here because the sample is incomplete. Could you provide a complete sample, including WindowArgs, to help us see what the issue is?

I don't have the project anymore but WindowArgs was just a non generic readonly record struct to pass in the creation args to the IWindow.

[jaredpar]

Updating the repro to the following:

public interface ICreatable<out T> where T : ICreatable<T>
{
    static abstract T Create();
}
public interface ICreatable<out T, in TArgs> where T : ICreatable<T, TArgs>
{
    static abstract T Create(TArgs args);
}

public interface IWindow : ICreatable<IWindow, WindowArgs>
{ 
    uint Width  { get; }
    uint Height { get; }
}

public record struct WindowArgs(int i) {}

Produces a very different error:

error CS8920: The interface 'IWindow' cannot be used as type argument. Static member 'ICreatable<IWindow, WindowArgs>.Create(WindowArgs)' does not have a most specific implementation in the interface.

The error difference is likely cause I'm using the C# 11 compiler vs you using the C# 10 compiler.

At a glance that error seems to make sense in this context but I will let @333fred say for sure.

[333fred]

This error is as-designed. Interfaces with static abstract members cannot be used as type arguments, as that presents a type safety hole that could cause a static member that does not actually exist to be invoked (causing a runtime exception).

[Shadowblitz16]

@333fred please reopen this. your failing to understand the issue.
The issue is that unlike instance interface members static interface members don't forward their requirements interfaces that implement them

public interface A
{
   public void Instance();
   public static abstract void Static();
}

public interface B : A //B  implements A so classes that implement B should be requires to implement A also
{
    
}

public class C : B // B should have the interface of A also but static members from A are not carried to B like instance members.
{
    
}

static interface members are not forwarded to other interfaces

[333fred]

@Shadowblitz16 looking at your example I'm not sure what you are saying. Can you show a us an example of where you expect to be able to use B? And to make sure, have you seen dotnet/csharplang#5955?

[Shadowblitz16]

@333fred sorry I am trying to be a clear as possible forgive my communication issues, I have autism so its hard for me.

basically the way I see static interface members is that they are just like instance members but are static.
I still don't understand the need for the abstract keyword and why normal static members aren't allowed but that's not the issue.

The issue is for example with instance members if B implements A and C implements B since B implements A, C is required to implement A also regardless if B redefineds A's members.

This doesn't seem to be the case with abstract static interface members.
with them it seems to act differently.

[333fred]

@Shadowblitz16 there is no difference there with static abstract and regular instance abstract members. Both will require that C satisfy all interfaces, in this case both A and B.

[Shadowblitz16]

@333fred I just want a static interface where I am not forced to implement it in the next interface or abstract class.

[CyrusNajmabadi]

@333fred I just want a static interface where I am not forced to implement it in the next interface or abstract class.

But then where will the implementation of that interface member be found?

[333fred]

@333fred I just want a static interface where I am not forced to implement it in the next interface

You are not forced to implement it in the next interface.

or abstract class.

The feature is not designed to allow this.