import std.experimental.all;

​

size_t myHashFunction(int a) { return cast(size_t) a; }

​

void main()

{

    auto b = new B!(int, myHashFunction);

}

​

template conditionMet(alias HashF)

{

    enum conditionMet = conditionOnInput!HashF && conditionOnOutput!HashF; 

}

​

template conditionOnInput(alias HashF)

{

    enum conditionOnInput = Parameters!HashF.length == 1 && is(Parameters!HashF[0] == int);

}

​

template conditionOnOutput(alias HashF)

{

    enum conditionOnOutput = is(ReturnType!HashF == size_t); 

}

​

class B(KeyT, alias HashF) if(conditionMet!HashF)

{ }