allow custom tokens to add extra logic contract via eip-1967 storage …

…slot

packages/ethereum-contracts/contracts/superfluid/SuperToken.sol

@@ -150,6 +150,39 @@ contract SuperToken is
         _initialize(underlyingToken, underlyingDecimals, n, s, adminOverride);
     }
 
+    // solhint-disable-next-line payable-fallback
+    fallback() external virtual {
+        _fallback();
+    }
+
+    // Hand to extended logic contract if specified in eip-1967-adjacent storage slot
+    function _fallback() internal virtual {
+        address extraImplementation;
+        assembly { // solium-disable-line
+            // eip-1967 storage slot decremented by 1
+            // bytes32(uint256(keccak256('eip1967.proxy.implementation')) - 2)
+            extraImplementation := sload(0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbb)
+        }
+
+        if (extraImplementation != address(0)) {
+            assembly {
+                // the following code is a copy (without comments) from openzeppelin Proxy
+                // see https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v4.9.3/contracts/proxy/Proxy.sol
+                calldatacopy(0, 0, calldatasize())
+                let result := delegatecall(gas(), extraImplementation, 0, calldatasize(), 0, 0)
+                returndatacopy(0, 0, returndatasize())
+
+                switch result
+                case 0 {
+                    revert(0, returndatasize())
+                }
+                default {
+                    return(0, returndatasize())
+                }
+            }
+        }
+    }
+
     function proxiableUUID() public pure virtual override returns (bytes32) {
         return keccak256("org.superfluid-finance.contracts.SuperToken.implementation");
     }