replace reinterpret_cast by unions in constexpr - good idea?

I am using this code to convert a pointer to a size_t, it is used in a ::std::hash function that should hash a given pointer at compile time, and since reinterpret_casts are not allowed in constexpr, I came up with the following solution, it works as expected but I want to know whether this can be considered a good idea. It would also be great if you could give me a few pointers on how portable this piece of code actually is.

union {
   size_t datr;
   void* pointer;
} dummy{.pointer = (void*) somePointer};
size_t pointerAsAsize_t = dummy.datr; // how portable is this?

Is there any better solution - as stated above, I want to create a ::std::hash that runs at compile time.

Thanks in advance, thejack