[Proposal]: Local generic type

[Kation]

Summary

C# should have a way to quickly use generic class and methods even though there is no generic argument but a type variable.

Design

C# should expose a syntax that allow developer to convert a type variable to a generic argument.

public class A<T> where T : IA
{
    public A(IB b) { }
    public A(IC c) { }
    public static void Init() { }
    public void Foo(IC c) { }
    public void Foo(ID d) { }
}

public interface IA { }
public interface IB { }
public interface IC { }
public interface ID { }

public class B
{
    public void M(Type type, IC c)
    {
        alias T where T : IA = type;
        A<T>.Init();
        A<T> a = new A<T>(c);
        A.Foo(c);
    }
}

Such as alias T = type;.
Also we can use generic constraints as alias T where T : IA = type;.

Then complier can generate codes:

public class B
{
    public void M(Type type, IC c)
    {
        I__GenericWrapper__1 wrapper = (I__GenericWrapper__1)Activator.CreateInstance(typeof(__GenericWrapper__1<>).MakeGenericType(type));
        wrapper.M1();
        wrapper.M2(c);
        wrapper.M3(c);
    }
    private interface I__GenericWrapper__1
    {
        void M1();
        void M2(IC c);
        void M3(IC c);
    }
    private class __GenericWrapper__1<T> : I__GenericWrapper__1
        where T : IA
    {
        private A<T> _a;
        public void M1()
        {
            A<T>.Init();
        }
        public void M2(IC c)
        {
            _a = new A<T>(c);
        }
        public void M3(IC c)
        {
            _a.Foo(c);
        }
    }
}

Also, we can have type check

checked alias T where T : IA = type;

Generate code:

if (!typeof(IA).IsAssignableFrom(type))
    throw;

[ufcpp]

#6308

[Kation]

It's a difference proposal.

[HaloFour]

See: #7916