This particular undefined case is a really interesting one; it seems utterly non-obvious why it exists, until you remember one thing... The most popular archit

This particular undefined case is a really interesting one; it seems utterly non-obvious why it exists, until you remember one thing... The most popular architecture today is 64-bit, with ABI specifying that the integer type is 32-bits wide. So when faced with performing an operation on two 32-bit signed integers, on a 64-bit platform, you have to either: 1) perform the operation in 32-bit registers - if available - and even if available, usually far far far fewer in number thus massive performance & resource penalties to code 2) perform the operation in 64-bit registers, then add code to check the result, and if it would overflow in 32-bit, compute what the overflowed result would have looked like in a 32-bit register, and return that instead - again quite a performance hit 3) perform the operation in 64-bit registers, and just return the lower 32-bits, whatever that might be The standard declined to pick a choice, which is why we have UB here. (For unsigned integers, you can just truncate the result and it works every time, which is why the standard defines the result)