unsigned x=1;
signed char y=-1;
x=1 : INT32:00000000 00000000 0000000 0000001
y=-1: signed char :11111111
提升: x=1 : INT32:00000000 00000000 0000000 0000001
y=-1: INT32 :1111111 11111111 11111111 11111111
转换: x=1U : INT32 : 00000000 00000000 0000000 0000001
y=UINT_MAX
: INT32 :1111111 11111111 11111111 11111111