Why is this call ambiguous?

Question

Can anyone explain, why does the following code produce the error? (Compiling in Microsoft Visual Studio 2008)

class Base1 {  };
class Base2 {  }

interface I1   {   }
interface I2   {   }

class C : I1, I2 { }

static class Program
{

    static T M1<T>(this T t, I1 x) where T : Base1
    {
        return t;
    }

    static T M1<T>(this T t, I2 x) where T : Base2
    {
        return t;
    }

    static void Main(string[] args)
    {
        Base1 b1 = new Base1();
        C c = new C();
        b1.M1(c);
    }
}

the error is

The call is ambiguous between the following methods or properties: 'ConsoleApplication1.Program.M1<ConsoleApplication1.Base1>(ConsoleApplication1.Base1, ConsoleApplication1.I1)' and 'ConsoleApplication1.Program.M1<ConsoleApplication1.Base1>(ConsoleApplication1.Base1, ConsoleApplication1.I2)'

I thought the compiler could distinguish between two methods using the "where" clauses

Answer 1

Constraints are not part of the signature for methods and thus are not used for resolution.