You can do that, until the compiler starts optimising, and then things go wrong.
Accessing any but the first element of a struct by using a pointer to the first element is undefined behaviour. "Undefined behaviour" means that anything can happen. The compiler may assume that there is no undefined behaviour.
There are many consequences that the compiler can deduce from that: If the compiler knows that your float* points to the first element of the struct, then it can deduce that every index to that array equals 0 (because anything else is undefined behaviour). If the compiler doesn't know this, then it can deduce that the array pointer cannot point to the struct, and changing array elements can't change struct elements and vice versa.
Can you see how this will go wrong?
