This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
In 126.96.36.199 [expr.unary.op], part of paragraph 7 describes how to compute the negative of an unsigned quantity:
The negative of an unsigned quantity is computed by subtracting its value from 2n, where n is the number of bits in the promoted operand. The type of the result is the type of the promoted operand.
According to this method, -0U will get the value 2n - 0 = 2n, where n is the number of bits in an unsigned int. However, 2n is obviously out of the range of values representable by an unsigned int and thus not the actual value of -0U. To get the result, a truncating conversion must be applied.
Rationale (April, 2007):
As noted in the issue description, a “truncating conversion” is needed. This conversion is supplied without need of an explicit mention, however, by the nature of unsigned arithmetic given in 6.8.2 [basic.fundamental] paragraph 4:
Unsigned integers, declared unsigned, shall obey the laws of arithmetic modulo 2n where n is the number of bits in the value representation of that particular size of integer.