ERC4626: Allow overriding underlying assets transfer mechanisms (#5970)

Author: Amxx

.changeset/spotty-plums-brush.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': minor
+---
+
+`ERC4626`: Allow overriding underlying assets transfer mechanisms through new internal virtual functions (`_transferIn` and `_transferOut`).

contracts/token/ERC20/extensions/ERC4626.sol

@@ -267,7 +267,7 @@ abstract contract ERC4626 is ERC20, IERC4626 {
         // Conclusion: we need to do the transfer before we mint so that any reentrancy would happen before the
         // assets are transferred and before the shares are minted, which is a valid state.
         // slither-disable-next-line reentrancy-no-eth
-        SafeERC20.safeTransferFrom(IERC20(asset()), caller, address(this), assets);
+        _transferIn(caller, assets);
         _mint(receiver, shares);
 
         emit Deposit(caller, receiver, assets, shares);
@@ -294,11 +294,21 @@ abstract contract ERC4626 is ERC20, IERC4626 {
         // Conclusion: we need to do the transfer after the burn so that any reentrancy would happen after the
         // shares are burned and after the assets are transferred, which is a valid state.
         _burn(owner, shares);
-        SafeERC20.safeTransfer(IERC20(asset()), receiver, assets);
+        _transferOut(receiver, assets);
 
         emit Withdraw(caller, receiver, owner, assets, shares);
     }
 
+    /// @dev Performs a transfer in of underlying assets. The default implementation uses `SafeERC20`. Used by {_deposit}.
+    function _transferIn(address from, uint256 assets) internal virtual {
+        SafeERC20.safeTransferFrom(IERC20(asset()), from, address(this), assets);
+    }
+
+    /// @dev Performs a transfer out of underlying assets. The default implementation uses `SafeERC20`. Used by {_withdraw}.
+    function _transferOut(address to, uint256 assets) internal virtual {
+        SafeERC20.safeTransfer(IERC20(asset()), to, assets);
+    }
+
     function _decimalsOffset() internal view virtual returns (uint8) {
         return 0;
     }