UFCS and overloading

[Szymon Gatner via Digitalmars-d-learn]

On Monday, 6 April 2015 at 17:53:13 UTC, Steven Schveighoffer 
wrote:
> On 4/6/15 12:23 PM, Szymon Gatner wrote:
>> Hi,
>>
>> I am surprised that this doesn't work:
>>
>> class Foo
>> {
>>   void bar(string) {}
>> }
>>
>> void bar(Foo foo, int i)
>> {
>> }
>>
>> auto foo = new Foo();
>> foo.bar(123); // <=== error
>>
>> causing compilation error:
>>
>> main.d(24): Error: function main.Foo.bar (string _param_0) is 
>> not
>> callable using argument types (int)
>>
>> does UFCS now work with method overloading? I know it is not a 
>> syntax
>> error because changing the name of int version of bar to bar2 
>> and
>> calling foo.bar2(123) works fine.
>
> You can't do this. UFCS cannot add overloads, it can only add 
> whole overload sets (if not already present).
>
> -Steve

Why is that? The use case is to provide a set of convenience 
"extension methods" to a basic interface. Say, given:

interface Subject
{
   void add(SomeInterface obj);
}

// and then
void add(Subject a, Type1 v1)
{
   a.add(convertToSomeInterface(v1));
}

void add(Subject a, Type2 v2)
{
   a.add(convertToSomeInterface(v2));
}

this way client can just implement Subject interface and still 
use it with types Type1 and Type2. C# allows that, why D does not?