Compile warning: negative left shifts

[physnoct]

home/user/.arduino15/packages/XMegaForArduino/hardware/avr/0.9.2/cores/xmega/HardwareSerial.cpp: In function 'uint16_t temp_get_baud(long unsigned int, uint8_t)':
/home/user/.arduino15/packages/XMegaForArduino/hardware/avr/0.9.2/cores/xmega/HardwareSerial.cpp:1063:20: warning: left shift of negative value [-Wshift-negative-value]
(uint16_t)(-1 << 12) | 137, // 57600

It goes on several lines.