Remove ERC20._burnFrom

[nventuro]

ERC20._burnFrom doesn't really fit with the standard for other internal functions, both in that it uses msg.sender and in that it is not really a reasonable primitive. It should disappear, and ERC20Burnable should implement burnFrom in terms of _burn and _approve.

[frangio]

I'm not sure this is as clear cut as you put it. If it's not a reasonable primitive, why is _transferFrom reasonable?

Tangentially to this specific issue, I'm starting to think perhaps having constraints on msg.sender in the internal functions would be the right thing. Should a contract be able to change completely the rules of the parent contracts?

[nventuro]

If it's not a reasonable primitive, why is _transferFrom reasonable?

It isn't, which is why it doesn't exist 😛

What do you mean by 'change the rules'? A contract writes their own rules, we simply provide ways for them to do so. I think having implicit arguments in these internal functions (msg.sender) makes it easy to misuse them, and this behavior can always be replicated by explicitly passing msg.sender, if needed.

[frangio]

What do you mean by 'change the rules'?

For example, we could say that one of the rules of ERC20 is that an account's balance can only be modified (by moving tokens elsewhere) through a transaction signed by the account owner themselves (or someone with allowance). By providing _transfer(from, to, amount) we are allowing a child contract of ERC20 to change that rule, allowing token movements to happen outside of those contraints. I understand this may be desirable in some cases, but in general I feel it might be better to fully encapsulate those rules in the parent contract.

[nventuro]

I believe that would land us on a design too stiff to be useful. IMO we should simply provide useful and correct primitives that can be used to build upon, as opposed to being too opinionated regarding what end user contracts should look like.

And even an invariant as basic as the transfer one would be broken by _mint and _burn, which are legitimate mechanisms used in multiple projects.