Remove the storage associated with decimals (#2502)

* Removing the storage associated with decimals

* changelog entry

* changelog link to new issue number

* Update contracts/token/ERC20/ERC20.sol

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

* Update contracts/token/ERC20/ERC20.sol

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

* Update CHANGELOG.md

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * Now targeting the 0.8.x line of Solidity compilers. For 0.6.x (resp 0.7.x) support, use version 3.4.0 (resp 3.4.0-solc-0.7) of OpenZeppelin.
 * `Context`: making `_msgData` return `bytes calldata` instead of `bytes memory` ([#2492](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2492))
+* `ERC20`: Removed the `_setDecimals` function and the storage slot associated to decimals. ([#2502](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2502))
 
 ## 3.4.0 (2021-02-02)
 

contracts/mocks/ERC20DecimalsMock.sol

@@ -5,7 +5,13 @@ pragma solidity ^0.8.0;
 import "../token/ERC20/ERC20.sol";
 
 contract ERC20DecimalsMock is ERC20 {
-    constructor (string memory name, string memory symbol, uint8 decimals) ERC20(name, symbol) {
-        _setupDecimals(decimals);
+    uint8 immutable private _decimals;
+
+    constructor (string memory name_, string memory symbol_, uint8 decimals_) ERC20(name_, symbol_) {
+        _decimals = decimals_;
+    }
+
+    function decimals() public view virtual override returns (uint8) {
+        return _decimals;
     }
 }

contracts/token/ERC20/ERC20.sol

@@ -38,13 +38,12 @@ contract ERC20 is Context, IERC20 {
 
     string private _name;
     string private _symbol;
-    uint8 private _decimals;
 
     /**
-     * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
-     * a default value of 18.
+     * @dev Sets the values for {name} and {symbol}.
      *
-     * To select a different value for {decimals}, use {_setupDecimals}.
+     * The defaut value of {decimals} is 18. To select a different value for
+     * {decimals} you should overload it.
      *
      * All three of these values are immutable: they can only be set once during
      * construction.
@@ -52,7 +51,6 @@ contract ERC20 is Context, IERC20 {
     constructor (string memory name_, string memory symbol_) {
         _name = name_;
         _symbol = symbol_;
-        _decimals = 18;
     }
 
     /**
@@ -76,15 +74,15 @@ contract ERC20 is Context, IERC20 {
      * be displayed to a user as `5,05` (`505 / 10 ** 2`).
      *
      * Tokens usually opt for a value of 18, imitating the relationship between
-     * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is
-     * called.
+     * Ether and Wei. This is the value {ERC20} uses, unless this function is
+     * overloaded;
      *
      * NOTE: This information is only used for _display_ purposes: it in
      * no way affects any of the arithmetic of the contract, including
      * {IERC20-balanceOf} and {IERC20-transfer}.
      */
     function decimals() public view virtual returns (uint8) {
-        return _decimals;
+        return 18;
     }
 
     /**
@@ -284,17 +282,6 @@ contract ERC20 is Context, IERC20 {
     }
 
     /**
-     * @dev Sets {decimals} to a value other than the default one of 18.
-     *
-     * WARNING: This function should only be called from the constructor. Most
-     * applications that interact with token contracts will not expect
-     * {decimals} to ever change, and may work incorrectly if it does.
-     */
-    function _setupDecimals(uint8 decimals_) internal virtual {
-        _decimals = decimals_;
-    }
-
-    /**
      * @dev Hook that is called before any transfer of tokens. This includes
      * minting and burning.
      *