More than one invariant per struct/class

[simendsjo]

I would like to use a template mixin to add some fields to a struct, but 
I'd also like the template to add additional invariant checks without 
having to remember to add this for all struct/classes that mixes in this 
code.

class C {
     int a;
}

mixin template EmbedC() {
     C _c;

     // oops.. more than one invariant
     invariant() {
         assert(_c);
     }

     void close() {
         _c = null;
     }
}

struct S {
     int i = 10;
     invariant() {
         assert(i >= 10);
     }

     mixin EmbedC!();
}

void main() {
     S s;
     s.close();
     s._c.a = 10; // access violation, but I want assertion from invariant
}