Generic parameter deduction

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Hi all,

I have a method where the compiler is unable to deduce the generic arguments. Now I'm asking myself if this a bug, or if not, why it isn't working. The method in question is Foo3:

using System;
using System.Collections.Generic;

namespace ConsoleApplication10
{
    class Program
    {
        class A<T> { }
        class B<T> { }

        static A<T> Foo1<T, AA>(AA a, T t) where AA : A<T>
        {
            return default(A<T>);
        }
        static A<T> Foo2<T, AA>(AA a, B<T> t) where AA : A<T>
        {
            return default(A<T>);
        }
        static A<T> Foo3<T, AA>(AA a) where AA : A<T>
        {
            return default(A<T>);
        }
        static void Main(string[] args)
        {
            A<int> ai = new A<int>();

            ai = Foo1(ai, 42);
            ai = Foo2(ai, new B<int>());

            // ... gives CS0411, can't deduce generic parameter. Why?
            ai = Foo3(ai);
        }
    }
}

The compiler can deduce T for Foo1, as it is explicitly defined as a parameter. It also can infer T & AA in Foo2, although T is only there implicitly as an generic argument.
But if it can handle Foo2, why not Foo3?

TIA,
Andy
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