# Re: [std-proposals] Reduce undefined behavior of signed integer literal arithmetic operations

From: Jonathan Wakely <cxx_at_[hidden]>
Date: Thu, 27 Apr 2023 20:56:28 +0100
On Thu, 27 Apr 2023 at 20:35, 萧 叶轩 via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
>
> Reduce undefined behavior of signed integer literal arithmetic operations
>
>
>
> Abstract
>
>
>
> Apply integral promotion on signed integer literal arithmetic operations
> to reduce undefined behavior.
> Background
>
> According to:
>
> basic.fundamental/1 <http://eel.is/c++draft/basic.fundamental#1> : The
> range of representable values for a signed integer type is −2<sup>N−1</sup>
> to 2<sup>N−1</sup> − 1.
>
> basic.fundamental/2 <http://eel.is/c++draft/basic.fundamental#2> :
> Overflow for signed arithmetic yields undefined behavior.
>
> expr.pre/4 <http://eel.is/c++draft/expr.pre#4> : If during the evaluation
> of an expression, the result is not mathematically defined or not in the
> range of representable values for its type, the behavior is undefined.
>
> Considering the following code, each line has an undefined behavior:
>
>
>
> auto a = INT_MAX + 1;
>
> auto b = -INT_MIN;
>
> long long c = INT_MAX + 1;
>
> long long d = -INT_MIN;
>
>
>
> GCC and Clang can diagnose that `INT_MAX + 1` and `-INT_MIN` have
> undefined behavior, while MSVC can only diagnose that `INT_MAX + 1` has.
> Solution
>
> Add a rule that when the operands of an operator are literals, apply
> integer promotion to increase the width of the type of the result to be
> large enough to store the value of the result value.
>

INT_MIN is not required to be a literal though, it might be something like
(-INT_MAX - 1). That's an integral constant expression of type int. So the
operand of -INT_MIN is not a literal.

None of the constants in <limits.h> are required to be literals, so INT_MAX
might be something like (2147483647), which also isn't a literal.