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Subject: Re: [std-proposals] Mistake in C++ Standard Arithmetic Conversion makes C++ compiler unreliable
From: Gašper Ažman (gasper.azman_at_[hidden])
Date: 2020-01-07 04:06:42


Did we inherit this from C?

On Tue, Jan 7, 2020 at 6:46 AM Владимир Прокофьев via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I have found a mistake in the C++ Standard,
> https://en.cppreference.com/w/cpp/language/operator_arithmetic#Conversion :
> in the rule:
>
> *Otherwise, if the unsigned operand's conversion rank is greater or equal
> to the conversion rank of the signed operand, the signed operand is
> converted to the unsigned operand's type.*
>
> The NEGATIVE signed value CANNOT be converted to unsigned for any types.
> But opposite conversion from unsigned to signed is valid with appropriate
> rank.
>
> To prove, please see wrong results here https://rextester.com/CKKDX46498
>
> Note, that the error is observed for VS, CLang and GCC C++ compilers.
>
> As you can see the error is observer in *division* operation to *unsigned*
> *32* and *64* bits.
> The *division* operation to *SIGNED* types is VALID (see *first* table in
> the example).
> Also *multiplication* of signed and unsigned values is VALID.
>
> The example above shows that the C++ compiler shows *unreliable* behavior
> which leads to produce wrong values as a result of division.
>
> The correct rule *shall* be:
>
>
> *Otherwise, unsigned operand is converted to signed type of same or higher
> conversion rank of unsigned operand's type and then the operand with lesser
> conversion rank is converted to the operand with the greater conversion
> rank*
> С уважением.
>
> Владимир.
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>



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