alt Hacker NewsSurely "We have many different ways to do this, each with different rules" is exactly the point? C++ 20's std::bit_cast isn't necessarily constexpr by the way although it is for the trivial byte <-> boolean transmutation I mentioned here.
I see that C++ people were more comfortable with the "We have far too many ways to initialize things" examples of this problem but I think transmutation hits harder precisely because it sneaks up on you.
bit_cast and reinterpret_cast do different things: one works at the value level, the second preserves address identity (and it is problematic from an aliasing point of view).
Not sure what any of this has to do with initialization though.
FWIW, the direct translation of your rust code is:
It fails on clang for y=2 and works for y=1, exactly like rust;GCC produces UB for y=2, I don't know if it is a GCC bug or the standard actually allows this form of UB to be ignored at contexpr time.
What is the rust equivalent of reinterpret_cast and does it work at constexpr time?
edit: I guess it would be an unsafe dereference of a casted pointer. Does it propagate constants?