Generic static extension member errs when used on a type parameter

[aradalvand]

Here's a realistic reproduction which yields:

var newId = TId.New(); // error CS0704: Cannot do non-virtual member lookup in 'TId' because it is a type parameter

There is no reason this should not be supported — as acknowledged by @CyrusNajmabadi here.

Expected Behavior:
The code should compile with no errors.

Actual Behavior:
It errs on TId.New().
Just lower the damn thing to IdExtensions.New<TId>(). What's so hard about that?

[MihuBot]

Possible related and/or duplicate issue:

• Extensions: Extension static member lookup on generic types does not work roslyn#78129

[bernd5]

Looks very odd to me, too. Especially because an explicit extension-member call would work as written by aradalvand:

using System;
using System.Numerics;

var s = new Something<IntId>();
s.DoSomething();

/// <summary>
/// Represents an opaque unique identifier type.
/// </summary>
public interface IId<TSelf> : IEquatable<TSelf>, IEqualityOperators<TSelf, TSelf, bool>
    where TSelf : struct, IId<TSelf>
{
    abstract static TSelf From(Guid guid);
    abstract static TSelf Parse(string guidAsString);

    abstract static implicit operator Guid(TSelf self);
}

record struct IntId(Guid guid) : IId<IntId>
{
    public static IntId From(Guid guid)
    {
        return new(guid);
    }

    public static IntId Parse(string guidAsString){
        return From(Guid.Parse(guidAsString));
    }

    public static implicit operator Guid(IntId self){
        return self.guid;
    }
}

public static class IdExtensions
{
    extension<T>(T) where T : struct, IId<T>
    {
        public static T New() => T.From(Guid.CreateVersion7());
    }
}

public sealed class Something<TId>
    where TId : struct, IId<TId>
{
    public void DoSomething()
    {
        //var newId = TId.New();
        var newId = IdExtensions.New<TId>();
        Console.WriteLine(newId);
    }
}

[jcouv]

We're not planning to relax this restriction in C# 14: https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-04-16.md#extension-static-member-lookup-on-generic-types-does-not-work