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

[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.

[quan-tob]

I tested this with concrete examples and found the original code (<) appears correct:

a	b	Original (<)	Proposed (>)	Actual
-10	5	OK ✓	OVERFLOW ✗	No overflow
MIN/2 - 1	2	OVERFLOW ✓	OK ✗	Overflow

When a < 0 and b > 0, underflow occurs when a * b < MIN, which means a < MIN / b (dividing by positive preserves inequality).

The proposed > causes false positives and misses real overflows.