Use _spendAllowance in ERC20FlashMint (#3226)

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

CHANGELOG.md

@@ -7,6 +7,7 @@
  * `ERC1155`: Add a `_afterTokenTransfer` hook for improved extensibility. ([#3166](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3166))
  * `DoubleEndedQueue`: a new data structure that supports efficient push and pop to both front and back, useful for FIFO and LIFO queues. ([#3153](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3153))
  * `Governor`: improved security of `onlyGovernance` modifier when using an external executor contract (e.g. a timelock) that can operate without necessarily going through the governance protocol. ([#3147](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3147))
+ * `ERC20FlashMint`: support infinite allowance when paying back a flash loan. ([#3226](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3226))
  * `Governor`: Add a way to parameterize votes. This can be used to implement voting systems such as fractionalized voting, ERC721 based voting, or any number of other systems. The `params` argument added to `_countVote` method, and included in the newly added `_getVotes` method, can be used by counting and voting modules respectively for such purposes.
 
 ### Breaking changes

contracts/token/ERC20/extensions/ERC20FlashMint.sol

@@ -73,9 +73,7 @@ abstract contract ERC20FlashMint is ERC20, IERC3156FlashLender {
             receiver.onFlashLoan(msg.sender, token, amount, fee, data) == _RETURN_VALUE,
             "ERC20FlashMint: invalid return value"
         );
-        uint256 currentAllowance = allowance(address(receiver), address(this));
-        require(currentAllowance >= amount + fee, "ERC20FlashMint: allowance does not allow refund");
-        _approve(address(receiver), address(this), currentAllowance - amount - fee);
+        _spendAllowance(address(receiver), address(this), amount + fee);
         _burn(address(receiver), amount + fee);
         return true;
     }

test/token/ERC20/extensions/ERC20FlashMint.test.js

@@ -67,7 +67,7 @@ contract('ERC20FlashMint', function (accounts) {
       const receiver = await ERC3156FlashBorrowerMock.new(true, false);
       await expectRevert(
         this.token.flashLoan(receiver.address, this.token.address, loanAmount, '0x'),
-        'ERC20FlashMint: allowance does not allow refund',
+        'ERC20: insufficient allowance',
       );
     });