Unused return value

Severity: Med ID: unused-return

Flags external calls whose return value is discarded, which often indicates a logic bug where the result of a computation or state query is silently ignored.

What it does

Detects high-level external calls (member calls on contract-typed variables or interface-cast addresses) that return one or more values when the entire result is discarded or any slot of a tuple return is omitted. ERC20 transfer and transferFrom are excluded as they are handled by the separate erc20-unchecked-transfer lint.

Why is this bad?

Discarding a return value can mask failures and incorrect assumptions. For example, ignoring the result of an oracle query or a state-mutating helper means the caller proceeds as if the call succeeded or that the value is irrelevant, both of which may be bugs.

Example

Bad

interface IOracle {
    function getPrice(address token) external returns (uint256);
}

contract Example {
    IOracle oracle;

    function updatePrice(address token) external {
        oracle.getPrice(token); // return value silently discarded
    }
}

Good

interface IOracle {
    function getPrice(address token) external returns (uint256);
}

contract Example {
    IOracle oracle;
    uint256 public lastPrice;

    function updatePrice(address token) external {
        lastPrice = oracle.getPrice(token);
    }
}