Adding signed numbers with opposite signs never generate overflow. Indeed, hardware usually has two flag bits related to addition, i.e., overflow bit and carry bit.
Overflow bit is only set if the container is not large enough to represent the number correctly. For signed number the hardware set this bit after analyzing the sign bit of each operand and the sign of the results. If the signs of operands are differents, it will never set this bit. Otherwise, if the signals of operands are equals, it analyses the sign of result. If it is different from the operands, it set this flag, indicating that the result is in overflow.
In your example, the sign of each operand is different, so there no overflow, but this operation generate a carry bit if you want to use large container to keep operation correct.