Or actually doing what they did here where it changes it to a

    enum Value {
         SimpleFloat(64bit value),
         ComplexNan(Heap pointer)
         Ptr(*const T)
    }

You lose out on performance if you use the bit patterns in the float that most people don't use very much, but you keep the correctness.

I'd be very unhappy if a compiler silently did this to me - it would make performance extremely hard to reason about. But it's not quite as bad as changing the semantics.