Document risk of SafeERC20 and ERC-7674 (#5262)

Signed-off-by: Hadrien Croubois <[email protected]>

Author: cairoeth

.changeset/yellow-tables-sell.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': minor
+---
+
+`SafeERC20`: Document risks of `safeIncreaseAllowance` and `safeDecreaseAllowance` when associated with ERC-7674.

contracts/token/ERC20/utils/SafeERC20.sol

@@ -46,6 +46,11 @@ library SafeERC20 {
     /**
      * @dev Increase the calling contract's allowance toward `spender` by `value`. If `token` returns no value,
      * non-reverting calls are assumed to be successful.
+     *
+     * IMPORTANT: If the token implements ERC-7674 (ERC-20 with temporary allowance), and if the "client"
+     * smart contract uses ERC-7674 to set temporary allowances, then the "client" smart contract should avoid using
+     * this function. Performing a {safeIncreaseAllowance} or {safeDecreaseAllowance} operation on a token contract
+     * that has a non-zero temporary allowance (for that particular owner-spender) will result in unexpected behavior.
      */
     function safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
         uint256 oldAllowance = token.allowance(address(this), spender);
@@ -55,6 +60,11 @@ library SafeERC20 {
     /**
      * @dev Decrease the calling contract's allowance toward `spender` by `requestedDecrease`. If `token` returns no
      * value, non-reverting calls are assumed to be successful.
+     *
+     * IMPORTANT: If the token implements ERC-7674 (ERC-20 with temporary allowance), and if the "client"
+     * smart contract uses ERC-7674 to set temporary allowances, then the "client" smart contract should avoid using
+     * this function. Performing a {safeIncreaseAllowance} or {safeDecreaseAllowance} operation on a token contract
+     * that has a non-zero temporary allowance (for that particular owner-spender) will result in unexpected behavior.
      */
     function safeDecreaseAllowance(IERC20 token, address spender, uint256 requestedDecrease) internal {
         unchecked {
@@ -70,6 +80,10 @@ library SafeERC20 {
      * @dev Set the calling contract's allowance toward `spender` to `value`. If `token` returns no value,
      * non-reverting calls are assumed to be successful. Meant to be used with tokens that require the approval
      * to be set to zero before setting it to a non-zero value, such as USDT.
+     *
+     * NOTE: If the token implements ERC-7674, this function will not modify any temporary allowance. This function
+     * only sets the "standard" allowance. Any temporary allowance will remain active, in addition to the value being
+     * set here.
      */
     function forceApprove(IERC20 token, address spender, uint256 value) internal {
         bytes memory approvalCall = abi.encodeCall(token.approve, (spender, value));