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Document safe usage of undefined behavior in gsl::narrow (#1024)

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dmitrykobets-msft 2022-01-26 16:44:07 -08:00 committed by GitHub
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@ -36,7 +36,12 @@ GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recogn
constexpr const bool is_different_signedness = constexpr const bool is_different_signedness =
(std::is_signed<T>::value != std::is_signed<U>::value); (std::is_signed<T>::value != std::is_signed<U>::value);
const T t = narrow_cast<T>(u); GSL_SUPPRESS(es.103) // NO-FORMAT: attribute // don't overflow
GSL_SUPPRESS(es.104) // NO-FORMAT: attribute // don't underflow
GSL_SUPPRESS(p.2) // NO-FORMAT: attribute // don't rely on undefined behavior
const T t = narrow_cast<T>(u); // While this is technically undefined behavior in some cases (i.e., if the source value is of floating-point type
// and cannot fit into the destination integral type), the resultant behavior is benign on the platforms
// that we target (i.e., no hardware trap representations are hit).
if (static_cast<U>(t) != u || (is_different_signedness && ((t < T{}) != (u < U{})))) if (static_cast<U>(t) != u || (is_different_signedness && ((t < T{}) != (u < U{}))))
{ {