Comment by flohofwoe

> Removing code paths that the programmer has explicitly laid out in the source code should be made a hard compile error unless the operation has been tagged with an attribute (anyone who wants to add the unsafe keyword to C? ).

Dead code elimination is essential for performance, especially when using templates (this is basically what enables the fabled "zero cost abstraction" because complex template code may generate a lot of 'inactive' code which needs to be removed by the optimizer).

The actual issue is that the compiler is free to eliminate code paths after UB, but that's also not trivial to fix (and some optimizations are actually enabled by manually injecting UB (like `__builtin_unreachable()` which can make a measurable difference in the right places).