template specialization and Rebindable

[vit via Digitalmars-d-learn]

Why is template param T != B but T == Rebindable!(const(B)) when 
specialization is T : const(A)?

abstract class A{}
class B : A{}
string foo(T : const(A))(T x){
    return T.stringof;
}

void main(){
    import std.typecons : Rebindable;
    import std.stdio : writeln;

    Rebindable!(const B) b = new B;

    auto bar = foo(b);
    assert(bar == "Rebindable!(const(B))");

}