Warnings.sol

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Warnings {

    // all the lines below will return a warning
    // ---
    // Warning: This declaration shadows a builtin symbol.

    string gasleft;
    uint tx;
    uint selfdestruct;
    
    // function keccak256() public pure;

    function warningExample1() pure internal {
        // Warning: Statement has no effect.
        5;
    }

    // Warning: Unnamed return variable can remain unassigned. 
    // Add an explicit return with value to all non-reverting code paths or name the variable.
    function warningExample2(uint _value) public pure returns (bool) {
        
        if ( _value < 10 ) {
            revert();
            // Warning: Unreachable code.
            _value = 12;
        }

    }

    uint256 a;
    
    // Warning: Function state mutability can be restricted to pure
    function warningExample3() public {
        // Warning: Unused local variable.
        uint256 a = 32;
    }

}