Question

Given a class that takes two policy template parameters:

template<typename PolicyA, typename PolicyB>
class widget;

And the following available policy classes A1, A2, A3, B1, B2, B3. How can convey that the 1s and 2s are compatible between each other but that A3 is only compatible with B3? That is, only the following instantiations are allowed:

widget<A1, B1> w11;    // All valid.
widget<A1, B2> w12;
widget<A2, B1> w21;
widget<A2, B2> w22;
widget<A3, B3> w33;

// No other combination allowed.

My failed attempt at using std::enable_if within a specialization was met with a compilation error:

template<typename A, typename B>
class<A3, enable_if<is_same<B, B3>::value, B3>::type>
{};

Answer 1

class A1; class A2; class A3; class B1; class B2; class B3;

/// Generic type
template <typename T1, typename T2>
class widget
{
  static_assert(std::is_same<T1, A3>::value ^ !std::is_same<T2, B3>::value,
    "Incompatible policy types selected.");
};

/// Some specialization
template <>
class widget<A1, A2>
{
};

widget<A1, B1> w11;
widget<A1, B2> w12;
widget<A2, B1> w21;
widget<A2, B2> w22;
widget<A3, B3> w33;
//widget<A1, B3> w13; // error C2338: Incompatible policy types selected.
//widget<A3, B2> w32; // error C2338: Incompatible policy types selected.