[dgrunwald]

> If a compiler is certain there's UB should it format the developer's hard drive immediately, or does it have to wait until the program is run before formatting to be standards compliant?

The standard has enough variants of UB that both options are possible!

For normal runtime UB (e.g. division by zero), it needs to wait until it is certain that the undefined code will actually be executed. So compilation/linking must not fail, the behavior is only undefined if the program is actually run.

IFNDR (ill-formed, no diagnostic required) is a different kind of UB; here it is expected that compilation/linking may already fail.

Then there's also the infamous "preprocessor UB" (e.g. https://wg21.link/p2621), which reuses the "undefined behavior" terminology despite not having anything to do with runtime semantics. I guess this is where the compiler might format your hard drive (old gcc versions actually launched nethack on undefined pragmas!).