Update ERC-7590: Handles erc20s with transfer fees.

Merged by EIP-Bot.

Author: steven2308

ERCS/erc-7590.md

@@ -179,6 +179,10 @@ Caution is advised when dealing with non-audited contracts.
 
 Implementations MUST use the message sender as from parameter when they are transferring tokens into an NFT. Otherwise, since the current contract needs approval, it could potentially pull the external tokens into a different NFT.
 
+When transferring [ERC-20](./eip-20.md) tokens in or out of an NFT, it could be the case that the amount transferred is not the same as the amount requested. This could happen if the [ERC-20](./eip-20.md) contract has a fee on transfer. This could cause a bug on your Token Holder contract if you do not manage it properly. There are 2 ways to do it, both of which are valid:
+1. Use the `IERC20` interface to check the balance of the contract before and after the transfer, and revert if the balance is not the expected one, hence not supporting tokens with fees on transfer.
+2. Use the `IERC20` interface to check the balance of the contract before and after the transfer, and use the difference to calculate the amount of tokens that were actually transferred. 
+
 To prevent a seller from front running the sale of an NFT holding [ERC-20](./eip-20.md) tokens to transfer out such tokens before a sale is executed, marketplaces MUST beware of the `erc20TransferOutNonce` and revert if it has changed since listed.
 
 [ERC-20](./eip-20.md) tokens that are transferred directly to the NFT contract will be lost.

assets/erc-7590/contracts/AbstractERC7590.sol

@@ -8,6 +8,7 @@ import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 error InvalidValue();
 error InvalidAddress();
 error InsufficientBalance();
+error InvalidAmountTransferred();
 
 abstract contract AbstractERC7590 is IERC7590 {
     mapping(uint256 tokenId => mapping(address erc20Address => uint256 balance))
@@ -57,10 +58,17 @@ abstract contract AbstractERC7590 is IERC7590 {
             amount,
             data
         );
+        IERC20 erc20 = IERC20(erc20Contract);
+        uint256 initBalance = erc20.balanceOf(address(this));
         _balances[tokenId][erc20Contract] -= amount;
         _erc20TransferOutNonce[tokenId]++;
 
-        IERC20(erc20Contract).transfer(to, amount);
+        erc20.transfer(to, amount);
+        uint256 newBalance = erc20.balanceOf(address(this));
+        // Here you can either use the difference as the amount, or revert if the difference is not equal to the amount and you don't want to support transfer fees
+        if (newBalance + amount != initBalance) {
+            revert InvalidAmountTransferred();
+        }
 
         emit TransferredERC20(erc20Contract, tokenId, to, amount);
         _afterTransferHeldERC20FromToken(
@@ -94,7 +102,14 @@ abstract contract AbstractERC7590 is IERC7590 {
             amount,
             data
         );
-        IERC20(erc20Contract).transferFrom(msg.sender, address(this), amount);
+        IERC20 erc20 = IERC20(erc20Contract);
+        uint256 initBalance = erc20.balanceOf(address(this));
+        erc20.transferFrom(msg.sender, address(this), amount);
+        uint256 newBalance = erc20.balanceOf(address(this));
+        // Here you can either use the difference as the amount, or revert if the difference is not equal to the amount and you don't want to support transfer fees
+        if (initBalance + amount != newBalance) {
+            revert InvalidAmountTransferred();
+        }
         _balances[tokenId][erc20Contract] += amount;
 
         emit ReceivedERC20(erc20Contract, tokenId, msg.sender, amount);