namespace Family1
{
    struct ModelA{};
    struct ModelB{};
};

namespace Family2
{
    struct ModelC{};
};

// valid for only classes in namespace Family1 only
template<typename T>
void Foo(T& _t ){}

Family1::ModelA modelA;
Family2::ModelC modelC;

Foo<Family1::ModelA>( modelA );     // OK
Foo<Family2::ModelC>( modelC );     // should cause a compile-time error