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Error in learn_evm/arithmetic-checks.md : Arithmetic checks for int256 multiplication #370

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@Strapontin

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@Strapontin

At this page : https://github.com/crytic/building-secure-contracts/blob/master/learn_evm/arithmetic-checks.md#arithmetic-checks-for-int256-multiplication

The first block of code is :

/// @notice versions >=0.8.0 && <0.8.17
function checkedMulInt(int256 a, int256 b) public pure returns (int256 c) {
    unchecked {
        c = a * b;

        if (a > 0 && b > 0 && a > type(int256).max / b) arithmeticError();
        if (a > 0 && b < 0 && a < type(int256).min / b) arithmeticError();
        if (a < 0 && b > 0 && a < type(int256).min / b) arithmeticError();
        if (a < 0 && b < 0 && a < type(int256).max / b) arithmeticError();
    }
}

There is an issue on the third condition that should be replaced :

/// @notice versions >=0.8.0 && <0.8.17
function checkedMulInt(int256 a, int256 b) public pure returns (int256 c) {
    unchecked {
        c = a * b;

        if (a > 0 && b > 0 && a > type(int256).max / b) arithmeticError();
        if (a > 0 && b < 0 && a < type(int256).min / b) arithmeticError();
-       if (a < 0 && b > 0 && a < type(int256).min / b) arithmeticError();
+       if (a < 0 && b > 0 && a > type(int256).min / b) arithmeticError();
        if (a < 0 && b < 0 && a < type(int256).max / b) arithmeticError();
    }
}

I discovered this issue while trying to use the code block for the solution of nodeGuardian's yul assembly quest.

Let me know if you need more details.

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