[simiones]

I suspect you are using "constexpr code/function" in a strange way. I believe most people use it to refer to code that is constexpr-qualified, such as "constexpr void foo() {...}". You seem to be using to refer to code that is a core constant expression, and that is where (most of) the misunderstanding comes from.

Still, it should be noted that even code that looks identical to a core constant expression can display certain kinds of UB if invoked at runtime, per the standard. For example, if the code is using certain STL classes or threading constructs in a way that is UB, it can still be considered a core constant expression. Additionally, the code may be well defined when used in a single-threaded context, but become UB when used in multi-threaded contexts, say if two threads are accessing an object member concurrently without the right barriers.

Here is an example on godbolt. Notice that bar() and baz() are defined the same way in the static_assert block and the runtime block, yet executing them constitutes UB in one case but not the other.

https://godbolt.org/z/3q5sTGe4j