Disallow virtual modifiers

[chriseth]

Modifiers are already a very complicated concept and we should not make it more difficult by allowing them to be virtual. Except for weird cases (adding another placeholder, putting it inside a loop, ...), you can get the same behaviour by adding a virtual function that is called from the modifier.

I would propose to remove this feature from the language with 0.9.0.

[0xpApaSmURf]

I personally quite like virtual modifiers. The separation in thinking as a developer treats functions differently from modifiers.

If you want to encourage the usage you suggested by using virtual functions inside modifiers, I would suggest just allowing functions to be able to decorate other functions and removing modifiers altogether:

contract MyContract {
    address owner;

    function onlyOwner() internal view {
        require(msg.sender == owner);
        _;
    }

    function performSensitiveOperation() external onlyOwner {
    ...
    }
}

This reduces the so-called "complicated modifiers" as a concept entirely and simply allows other functions to become pre-cursor conditions to the "main" function body.

If the idea of decorating a function with a modifier is the complication, then the simplest alternative is to stop allowing modifiers other than built-in visibility modifiers and force people to use functional conditions at the beginning of functions instead of modifiers.