[kazinator]

> b and c both fit in 31 bits

They don't fit into a 31 bit two's complement (i.e. signed) representation, in terms of representing their interpretation as the familiar 32 bit INT_MAX.

31 bit two's complement goes from -0x40000000 to 0x3FFFFFFF. There is a 0x7FFFFFFF bit pattern, which represents -0x00000001. It has a sign bit which is 1. (So, adding that to itself does go to -2, but under that interpretation there is no overflow.)

Any pair of values in that range can be added or subtracted in 32 bit two's complement.

Including the most negative value: -0x40000000 + -0x40000000 = -0x80000000.