Incorrect using-for

Severity: Info ID: incorrect-using-for

Flags using L for T directives whose library has no function applicable to the type: they attach nothing.

What it does

Reports a using ... for directive, file-level or contract-level, when no function of the named library accepts the target type as its bound first parameter. Attachment follows the type checker, implicit conversions included: a uint8 binds to a uint256 parameter and a derived contract to a base parameter. Reference types are probed under every data location their bound value may live in (storage, memory and calldata), so a library whose only applicable function takes a calldata first parameter counts as attaching. Private library functions never count: the library form skips them, even when a braced directive in scope attaches one. using L for * and the braced form using {f} for T are out of scope, the latter because the compiler already rejects a function that cannot attach. Inspired by Slither's incorrect-using-for detector, with a different scope: contract-level directives are covered, attachment follows Solar's member resolution across data locations, and private functions are excluded.

Why is this bad?

A directive that attaches nothing is dead code, and usually a typo: the wrong library, or the wrong type. The compiler accepts it silently, so the mistake surfaces later as a confusing Member "f" not found error at the call site, or never surfaces at all.

Example

Bad

library CounterLib {
    function increment(uint256 v) internal pure returns (uint256) {
        return v + 1;
    }
}

contract C {
    // no function of CounterLib takes an address
    using CounterLib for address;
}

Good

contract C {
    using CounterLib for uint256;

    uint256 internal counter;

    function bump() internal view returns (uint256) {
        return counter.increment();
    }
}