Accessing constructors with UnsafeAccessor throws exception on generic types in .NET9

[yushroom]

Description

Accessing constructors with UnsafeAccessor throws exception on generic types in .NET9 preview 6.

Reproduction Steps

using System.Runtime.CompilerServices;

class Program
{
    static List<int> CallConstructorByUnsafeAccessor()
    {
        var c = (List<int>)RuntimeHelpers.GetUninitializedObject(typeof(List<int>));
        CtorAsMethod(c);
        return c;

        [UnsafeAccessor(UnsafeAccessorKind.Method, Name = ".ctor")]
        extern static void CtorAsMethod(List<int> c);
    }

    static void Main()
    {
        var c = CallConstructorByUnsafeAccessor();
        Console.WriteLine(c.Count);
    }
}

Expected behavior

The code works on .NET 8 and .NET 9

Actual behavior

• No error on .NET 8
• Fails on .NET 9 preview 6

Unhandled exception. System.InvalidProgramException: Generic type constraints do not match.

Also fails with NativeAOT

dotnet publish -r win-x64 -c Debug /p:PublishAot=true

EXEC : error : Index was outside the bounds of the array.

Regression?

Yes as it works on .NET 8

Known Workarounds

No response

Configuration

No response

Other information

No response

[jkotas]

cc @AaronRobinsonMSFT

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[AaronRobinsonMSFT]

So most generic support was deemed broken in .NET 8. I tried to capture that in #89439. It clearly wasn't properly expressed nor was the fact that generic parameters now must match in location and number. The documentation for UnsafeAccessor was recently updated specifically talking about generics.

For context, this was part of type parameters must be type parameters and method parameters must be method parameters so signature matching was simpler. The result is that we match signatures precisely. For the above scenario it is relatively simple and we could make it work, but it quickly becomes much more complicated when generic parameters are involved. I'm inclined to report a breaking change issue here for .NET 8 instead.

The workaround is to follow the supported generic way to handle this:

    static List<int> CallConstructorByUnsafeAccessor()
    {
        var c = (List<int>)RuntimeHelpers.GetUninitializedObject(typeof(List<int>));
        Accessor<int>.CtorAsMethod(c);
        return c;
    }

    class Accessor<T>
    {
        [UnsafeAccessor(UnsafeAccessorKind.Method, Name = ".ctor")]
        public extern static void CtorAsMethod(List<T> c);
    }

    static void Main(string[] args)
    {
        var c = CallConstructorByUnsafeAccessor();
        Console.WriteLine(c.Count);
    }

[AaronRobinsonMSFT]

I'm inclined to report a breaking change issue here for .NET 8 instead.

Re-reading through the history of the original feature PR, specifically here, this sounds like this could be considered a bug. I genuinely don't think that is fair so think a breaking change issue is more appropriate because it wasn't blocked in .NET 8.

[AaronRobinsonMSFT]

After some additional discussion, we've decide to document this as a breaking change. It can be tracked at dotnet/docs#41769.