Parameter specialization

[Eyyub]

Hello,

I have a question about the semantic of parameter 
specialization(TemplateTypeParameterSpecialization)
In this following code, there are 2 forms of the same function 
'add' :
<code>

T add(T, U : T) (T a, U b)   //doesn't work
{
     return a + b;
}

T add(T, U) (T a, U b) if(is(U : T)) //works
{
     return a + b;
}


void main()
{
     assert(add(2, cast(short)2) == 4);
}
</code>
So, I infer that, in this case, 
TemplateTypeParameterSpecialization and TypeSpecialization(of 
IsExpression) aren't semantically equal ? What are differences 
between this 2 forms ?

Eyyub,

(I hope that I have an english a bit compréhensible)