This diagnostic rule is based on the software development guidelines developed by AUTOSAR (AUTomotive Open System ARchitecture).
As specified by the C++ standard, an overflow of values of unsigned types results in a wrap-around. Using this mechanism in evaluation of expressions at runtime is a well-known practice (unlike signed types, where an overflow leads to undefined behavior).
However, an unsigned integer wrap-around in expressions evaluated at compile time may be misleading.
Example of non-compliant code:
#include <stdint.h>
#define C1 (UINT_MAX)
#define C2 (UINT_MIN)
....
void foo(unsigned x)
{
switch(x)
{
case C1 + 1U: ....; break;
case C2 - 1U: ....; break;
}
}According to this rule, an unsigned integer wrap-around that occurs when evaluating a constant expression of unsigned type, it will not be treated as an error if the expression will never be evaluated:
#include <stdint.h>
#define C UINT_MAX
....
unsigned foo(unsigned x)
{
if(x < 0 && (C + 1U) == 0x42) ....;
return x + C;
}The '(C + 1U)' expression resulting in an overflow will not be executed since the 'x < 0' condition is always true. Therefore, the second operand of the logical expression will not be evaluated.
This diagnostic is classified as:
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