A very neat thing of constexpr is that it does not have undefined behaviour ; any thing that would cause UB at run-time is flagged as a compile error. Every variable must be fully initialized.

You simply can't. The constexpr environment won't let you have uninitialized variables/memory anywhere. It won't let you access dangling/out-of-bounds pointers either, overflow two-complement types, etc.

Accessing uninitialized objects is undefined behaviour, which traps in the constexpr context.