Local variables used prior their declaration SLITHER-W1056

Detects the possible usage of a variable before the declaration is stepped over (either because it is later declared, or declared in another scope).

Exploit Scenario

contract C {
    function f(uint z) public returns (uint) {
        uint y = x + 9 + z; // 'x' is used pre-declaration
        uint x = 7;

        if (z % 2 == 0) {
            uint max = 5;
            // ...
        }

        // 'max' was intended to be 5, but it was mistakenly declared in a scope and not assigned (so it is zero).
        for (uint i = 0; i < max; i++) {
            x += 1;
        }

        return x;
    }
}

In the case above, the variable x is used before its declaration, which may result in unintended consequences. Additionally, the for-loop uses the variable max, which is declared in a previous scope that may not always be reached. This could lead to unintended consequences if the user mistakenly uses a variable prior to any intended declaration assignment. It also may indicate that the user intended to reference a different variable.

Recommendation

Move all variable declarations prior to any usage of the variable, and ensure that reaching a variable declaration does not depend on some conditional if it is used unconditionally.

Learn more

variable-scope on Slither's wiki.