Adding support for open generic types in attributes

[hez2010]

Enable open generic types in attributes.

Passing generic context in S.P.CoreLib so that open generic types can be resolved.
Skip instantiating generic attribute type if any type parameter is not provided.

Language proposal: dotnet/csharplang#8198

Pseudo C# code:

using System;
using System.Reflection;

class Program
{
    static void Main()
    {
        Console.WriteLine("Type");
        var type = typeof(Foo<int>);
        foreach (var attr in type.GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Generic Type Parameter");
        foreach (var attr in type.GetGenericTypeDefinition().GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Method");
        var method = type.GetMethod("Bar").MakeGenericMethod(typeof(bool));
        foreach (var attr in method.GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Param");
        foreach (var param in method.GetParameters())
        {
            foreach (var attr in param.GetCustomAttributes())
            {
                Console.WriteLine(attr);
            }
        }
        Console.WriteLine("Generic Method Parameter");
        foreach (var genericParam in method.GetGenericMethodDefinition().GetGenericArguments())
        {
            foreach (var attr in genericParam.GetCustomAttributes())
            {
                Console.WriteLine(attr);
            }
        }
        Console.WriteLine("Property");
        var prop = type.GetProperty("Prop");
        foreach (var attr in prop.GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Property Getter");
        foreach (var attr in prop.GetGetMethod().GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Property Setter");
        foreach (var attr in prop.GetSetMethod().GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Field");
        var field = type.GetField("Field");
        foreach (var attr in field.GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Event");
        var eventInfo = type.GetEvent("Event");
        foreach (var attr in eventInfo.GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Event Adder");
        foreach (var attr in eventInfo.GetAddMethod().GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
        Console.WriteLine("Event Remover");
        foreach (var attr in eventInfo.GetRemoveMethod().GetCustomAttributes())
        {
            Console.WriteLine(attr);
        }
    }
}

[My<T, T>]
class Foo<[My<T, T>] T>
{
    [My<T, U>]
    public void Bar<[My<T, U>] U>([My<T, U>] T value)
    {
    }

    [My<T, T>]
    public int Prop { [My<T, T>] get; [My<T, T>] set; }
    
    [My<T, T>]
    public int Field;

    [My<T, T>]
    public event Action<int> Event { [My<T, T>] add { } [My<T, T>] remove { }}
}

class MyAttribute<T, U> : Attribute
{
    public T Value1 { get; } = default;
    public U Value2 { get; } = default;

    public override string ToString()
    {
        return typeof(MyAttribute<T, U>) + " " + Value1 + " " + Value2;
    }
}

IL for testing: il.txt

Output:

Type
MyAttribute`2[System.Int32,System.Int32] 0 0
Generic Type Parameter
// no output here as the generic context of generic args returned from `type.GetGenericTypeDefinition()` has been stripped
Method
MyAttribute`2[System.Int32,System.Boolean] 0 False
Param
MyAttribute`2[System.Int32,System.Boolean] 0 False
Generic Method Parameter
// no output here as the generic context of generic args returned from `method.GetGenericMethodDefinition().GetGenericArguments()` has been stripped
Property
MyAttribute`2[System.Int32,System.Int32] 0 0
Property Getter
MyAttribute`2[System.Int32,System.Int32] 0 0
System.Runtime.CompilerServices.CompilerGeneratedAttribute
Property Setter
MyAttribute`2[System.Int32,System.Int32] 0 0
System.Runtime.CompilerServices.CompilerGeneratedAttribute
Field
MyAttribute`2[System.Int32,System.Int32] 0 0
Event
MyAttribute`2[System.Int32,System.Int32] 0 0
Event Adder
MyAttribute`2[System.Int32,System.Int32] 0 0
Event Remover
MyAttribute`2[System.Int32,System.Int32] 0 0

Tests will be added later.

[jkotas]

This is a partial prototype for a runtime support of a proposed C# language feature. There is not much to review. We should agree first that this is a C# language feature that we want to build.

[ericstj]

Per @jkotas comment, closing this PR as it's not ready for merge.