How is opEquals used in toHash

[PinDPlugga]

In the solution to one of the exercises in Programming in D the 
unittests fail with respect to the toHash implementation.

Here is a link to the full solution provided:
https://run.dlang.io/gist/99ddf791f86aaa9d333d032166aadcb9?args=-unittest%20-main

and the link to the relevant section in the book:
https://ddili.org/ders/d.en/object.html

so while the unittest for equality passes, the next test with 
`in` is where the failure occurs:
```D
// ...
     assert(area1 == area2);    // Passes

     double[TriangularArea] areas;
     areas[area1] = 1.25;

     assert(area2 in areas); // Fails
// ...
```

the issue must be with the toHash function given
``` D
     override size_t toHash() const {
         /* Since the 'points' member is an array, we can take
          * advantage of the existing toHash algorithm for
          * array types. */
         return typeid(points).getHash(&points);
     }
```

I looked into the object library documentation and saw that the 
template getHash calls hashOf which calls 
core.internal.hash.hashOf etc.

But what I do not understand is why opEquals is necessary and 
where in the implementation of toHash it plays its role? Since 
area1 and area2 have different static arrays of Points I 
understand why `typeid(points).getHash(&points)` would produce 
different values for each, but it seems like the opEquals should 
play a part in making them produce the same hash.