Add an internal _useNonce(address) function in ERC20Permit (#2565)

CHANGELOG.md

@@ -4,6 +4,7 @@
 
  * `IERC20Metadata`: add a new extended interface that includes the optional `name()`, `symbol()` and `decimals()` functions. ([#2561](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2561))
  * `ERC777`: make reception acquirement optional in `_mint`. ([#2552](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2552))
+ * `ERC20Permit`: add a `_useNonce` to enable further usage of ERC712 signatures. ([#2565](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2565))
 
 ## Unreleased
 

contracts/token/ERC20/extensions/draft-ERC20Permit.sol

@@ -47,7 +47,7 @@ abstract contract ERC20Permit is ERC20, IERC20Permit, EIP712 {
                 owner,
                 spender,
                 value,
-                _nonces[owner].current(),
+                _useNonce(owner),
                 deadline
             )
         );
@@ -57,14 +57,13 @@ abstract contract ERC20Permit is ERC20, IERC20Permit, EIP712 {
         address signer = ECDSA.recover(hash, v, r, s);
         require(signer == owner, "ERC20Permit: invalid signature");
 
-        _nonces[owner].increment();
         _approve(owner, spender, value);
     }
 
     /**
      * @dev See {IERC20Permit-nonces}.
      */
-    function nonces(address owner) public view override returns (uint256) {
+    function nonces(address owner) public view virtual override returns (uint256) {
         return _nonces[owner].current();
     }
 
@@ -75,4 +74,13 @@ abstract contract ERC20Permit is ERC20, IERC20Permit, EIP712 {
     function DOMAIN_SEPARATOR() external view override returns (bytes32) {
         return _domainSeparatorV4();
     }
+
+    /**
+     * @dev "Consume a nonce": return the current value and increment.
+     */
+    function _useNonce(address owner) internal virtual returns (uint256 current) {
+        Counters.Counter storage nonce = _nonces[owner];
+        current = nonce.current();
+        nonce.increment();
+    }
 }