Justification

Unary minus on an unsigned expression gives an unsigned result which is never negative; unary minus on a non-arithmetic expression is not allowed.

Example

long j;
unsigned int i = 1;
signed int k = 1;
...
j = -i; /* WRONG: very large positive number */
j = -k; /* RIGHT */

Note

The negative is taken after any necessary promotions have been applied to the operand.