ERC20 totalSupply changed from public property to a function

[spalladino]

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented.
• 🎨 I've run the JavaScript linter (npm run lint:fix) and fixed all issues.

Fixes #434

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🚀 Description

Having ERC20#totalSupply defined as a public property made it impossible to override it in any child class, due to a solidity limitation. As such, it was impossible to change the getter behaviour while retaining the ERC20 interface, at least without modifying the OpenZeppelin base contract (which is something we definitely want to avoid).

This PR removes the public property definition from the ERC20Basic interface, and replaces it with just the signature for the getter. The implementation is provided in BasicToken, the first contract that implements that interface, backed by a totalSupply_ internal field.

Regarding the naming convention for the backing field: since we are using leading underscores for the parameters, I unilaterally decided to use trailing underscore for the backing fields. Other options I considered were using a dollar sign ($totalSupply or totalSupply$), or some variant of Hungarian notation (u256TotalSupply).

[spalladino]

Heh, PR 666 😈

[facuspagnuolo]

I thought we had said that we were going to use an underscore as a prefix, instead of a suffix, am I right?

[spalladino]

Yeah, hence the last comment on the PR:

Regarding the naming convention for the backing field: since we are using leading underscores for the parameters, I unilaterally decided to use trailing underscore for the backing fields. Other options I considered were using a dollar sign ($totalSupply or totalSupply$), or some variant of Hungarian notation (u256TotalSupply).

[frangio]

Not happy with the trailing underscore notation but cannot think of a better alternative.

[spalladino]

@frangio please re-review :-)

[hlogeon]

I got a compilation error because of this. It tells the following:

zeppelin-solidity/contracts/token/ERC20/ERC20Basic.sol:10:3: TypeError: Redeclaring an already implemented function as abstract
  function totalSupply() public view returns (uint256);
  ^---------------------------------------------------^
,zeppelin-solidity/contracts/token/ERC20/ERC20Basic.sol:10:3: TypeError: Redeclaring an already implemented function as abstract
  function totalSupply() public view returns (uint256);
  ^---------------------------------------------------^

[frangio]

Where did that happen @hlogeon? If you can reproduce would you mind opening a new issue please?