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TIL! I went and dug up a citation: https://blog.reverberate.org/2013/08/parsing-c-is-literally-...

If you don't set the underlying type, assigning a value that doesn't match an enumerator via `static_cast` is undefined behavior. See https://en.cppreference.com/w/cpp/language/enum . (Doing weird pointer casting things is also undefined behavior per the strict aliasing rule, though, come to think of it, I'm not sure whether memcpying an out-of-range value into an enum through the "reinterpret_cast to `char*`" loophole is undefined behavior.)

The article links out to Compiler Explorer (https://godbolt.org/), which shows the exact compiler version and options and lets you play with different ones. It shouldn’t matter which compiler — the standard prohibits optimizing this.