It was undefined what happens at unsigned overflows and underflows. Therefore a compiler could choose to implement "unsigned" as either non-negative numbers or as integer residues.
The fact that "sizeof" is unsigned and the implicit conversions between "unsigned" numbers are consistent only with non-negative numbers. Therefore the undefined behavior should have been defined correspondingly.
Instead of this, at some version of the standard, I am lazy to search it now, but it might have been C99, they have changed the behavior from undefined to defined as the behavior of integer residues.
I do not know the reason for this choice, it may have been just laziness, because it is easier to implement in compilers and it leads to maximum performance in the absence of bugs. In any case this decision has broken the standard, because the arithmetic operations have become incompatible with the implicit conversions between "unsigned" types and with the semantics of "sizeof", which must be non-negative.
For non-negative numbers, the correct conversions are from smaller sizes to bigger sizes, while for integer residues the correct conversions are only in the opposite direction, from bigger sizes to smaller sizes (e.g. a number that is 257 modulo 65536 is also 1 modulo 256, so truncating it yields a correct value, while a number that is 1 modulo 256 when modulo 65536 it could be 257, 511, 769 etc. so you cannot extend it without additional information).
Judging from the implicit conversions, it is clear that the intention of the designers of C during the seventies was that "unsigned" numbers must be non-negative integers and not integer residues. The modern C standard is guilty of the current inconsistencies that greatly increase the chances of bugs
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Nope.
It was undefined what happens at unsigned overflows and underflows. Therefore a compiler could choose to implement "unsigned" as either non-negative numbers or as integer residues.
The fact that "sizeof" is unsigned and the implicit conversions between "unsigned" numbers are consistent only with non-negative numbers. Therefore the undefined behavior should have been defined correspondingly.
Instead of this, at some version of the standard, I am lazy to search it now, but it might have been C99, they have changed the behavior from undefined to defined as the behavior of integer residues.
I do not know the reason for this choice, it may have been just laziness, because it is easier to implement in compilers and it leads to maximum performance in the absence of bugs. In any case this decision has broken the standard, because the arithmetic operations have become incompatible with the implicit conversions between "unsigned" types and with the semantics of "sizeof", which must be non-negative.
For non-negative numbers, the correct conversions are from smaller sizes to bigger sizes, while for integer residues the correct conversions are only in the opposite direction, from bigger sizes to smaller sizes (e.g. a number that is 257 modulo 65536 is also 1 modulo 256, so truncating it yields a correct value, while a number that is 1 modulo 256 when modulo 65536 it could be 257, 511, 769 etc. so you cannot extend it without additional information).
Judging from the implicit conversions, it is clear that the intention of the designers of C during the seventies was that "unsigned" numbers must be non-negative integers and not integer residues. The modern C standard is guilty of the current inconsistencies that greatly increase the chances of bugs