Demonstration in https://chromium-review.googlesource.com/c/575971/ - this test cases fails with triggering __builtin_trap(), I believe when the HashMap compares zero initialized memory to find an empty bucket, but I am not exactly sure.

It would be great if you had any suggestions as to how to address this. What to modify in the HashTraits for example.

I guess CheckedNumeric::ValueOrDie() called in ClassUsingCheckedInt16::operator== dies because CheckedNumericState::is_valid_ is 0. kEmptyValueIsZero in HashTraits should be false.

It's unclear if we want to do this... Generally, storing as int would be
sufficient (but be careful that 0 and -1 are not usable).

One extra bool flag which should be always true simply makes the space
efficiency worse.

One possible idea is to store the actual data as a plain int, ignoring the
CheckedNumeric data structure. Getters and setters can only receive
CheckedNumeric<Int>. However, it's not clear whether we can make 0 and -1 as
the empty value and the deleted value.

I have no idea about how useful such traits would be...

Thanks for taking the time to take a look and providing feedback. What you're saying partially overlaps with what I heard from Justin: It's not efficient to use a CheckedNumeric for storage, but it should instead be mainly used for computation.