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Выдержки из стандарта N3690 (4.13, 5.10):
"The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type."
"Otherwise, if the operand that has unsigned integer type has rank greater than or equal to the
rank of the type of the other operand, the operand with signed integer type shall be converted to
the type of the operand with unsigned integer type."

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http://dariaemacs.com/types#tasks