Solidity MacOSX – Stuck Installing Solidity with Brew

The abstract keyword has been introduced in the Solidity version 0.6.0.

Just update your contract from version 0.5.0 to 0.6.0 or, if you can't, change it to be compatible with the 0.5.0 release. This page about the breaking changes between the two releases could help you a lot : (https://docs.soliditylang.org/en/v0.6.0/060-breaking-changes.html).

Note that the pragma directive used by the 0penZeppelin Ownable contract is pragma solidity >=0.6.0 <0.8.0;. If you use a previous Solidity release you should expect some errors in your code.

The answer was given to me on reddit so here is the fixed contract for anyone who may need it :D :

// SPDX-License-Identifier: MIT pragma solidity 0.8.0;

contract Foo {
    // I can't change the parameters of this function but I can change the content
    function bar (bytes calldata data) public {
        (uint a, bytes memory b, bytes memory c) = abi.decode(data, (uint, bytes, bytes));
        this.qux(c);
    }
    
    // I can't change this function at all
    function qux (bytes calldata data) public {
        // Do something here
    }
    
    // I can change this function, the parameters are just an example
    function baz (uint _a, bytes calldata _b, bytes calldata _c) public {
        uint a = _a;
        bytes calldata b = _b;
        bytes calldata c = _c;
        
        bytes memory data = abi.encode(
            a,
            b,
            c
        );
    
        this.bar(data);
    }
}