why do extension functions ignore the where clause?

[not_a_commie]

I have here some example code (below) that I believe should work fine.
However, I get the error

Error 1 The call is ambiguous between the following methods or
properties:
'TestExtensions.ExtensionsOne.AddFive<TestExtensions.One>(TestExtensions.One)'
and
'TestExtensions.ExtensionsTwo.AddFive<TestExtensions.One>(TestExtensions.One)'
C:\workspace\TestProjects\TestExtensions\TestExtensions\Program.cs
44 4 TestExtensions

It's as though the resolution of the extension functions ignores the
where clause on the functions. As a workaround I can call the function
statically. I just think this should be resolvable.

namespace TestExtensions
{
public interface IOne
{
int Int { get; set; }
}

public class One: IOne
{
public int Int { get; set; }
}

public interface ITwo
{
int Int { get; set; }
}

public class Two : IOne
{
public int Int { get; set; }
}

public static class ExtensionsOne
{
public static void AddFive<T>(this T obj) where T : IOne
{
obj.Int += 5;
}
}

public static class ExtensionsTwo
{
public static void AddFive<T>(this T obj) where T : ITwo
{
obj.Int += 5;
}
}

class Program
{
static void Main(string[] args)
{
var one = new One();
one.AddFive();
}
}
}

 

[not_a_commie]

I should add that using classes instead of interfaces in the where
clause makes no improvement to the problem.

 

[Jon Skeet [C# MVP]]

I have here some example code (below) that I believe should work fine.

Why do you believe it should work? I can't see anything in the overload
resolution section of the spec (7.4.3.3 is the relevant bit here, I
believe) which suggests that generic constraints should make one
conversion better than another.