Generics stop compiling when switching from .NET 4 --> NET 3.5 / Silverlight 4

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C#

C#4.0

C#3.5

Silverlight4

Hi everyone,
I'm in the process of putting a massive amount of c# code originally written for .NET 4.0 into a silverlight 4 app. I have quite a serious problem with the compiler rejecting generics when inheritance is involved. I've put a very basic example down below: this code compiles OK in .NET 4, but not in silverlight 4.0 or .NET 3.5. Can someone with more experience please tell me what's going on here?? I surely can't be the first to come across such issues:

using System;
using System.Collections.Generic;

namespace ClassLibrary2
{
    public class Main<T>
        where T : Class1
    {
        public void DoStuff()
        {
            //The following line compiles in .NET4 but does not compile in Silverlight 4
            //Error	2	Argument 1: cannot convert from 'ClassLibrary2.ComparingClass' to 'System.Collections.Generic.IComparer<T>'
            Class2<T, IComparer<T>> returnVal = new Class2<T, IComparer<T>>(new ComparingClass());
        }
    }

    public class Class1
    {
    }

    class Class2<T, U>
        where T : Class1
        where U : IComparer<T>
    {
        internal Class2(U c)
        {
        }
    }

    class ComparingClass : IComparer<Class1>
    {
        public int Compare(Class1 a, Class1 b)
        {
            return 1;
        }
    }
}

I have similar problems with things inheriting from List<t> not getting access to LINQ IEnumerable<t> extension methods etc. All of this code compiled A-OK in .NET 4...
Any help will be greatly appreciated.

EDIT:

I've found a solution *to this example*:
change

class ComparingClass : IComparer<Class1>

to

class ComparingClass<T> : IComparer<T>
        where T:Class1

but in general I don't understand why the compiler thinks of them differently. I have a lot of code to move across with similar issues and need to understand what's going wrong in order to fix it. Help!