Fallback function of Proxy contraact

Proxy.js

     function ()
     payable
     external {
         address implementation = Resolver(resolver()).getUserVersion(msg.sender);
         require(implementation != address(0), "INVALID IMPLEMENTATION");
         assembly {
             let ptr := mload(0x40)
             calldatacopy(ptr, 0, calldatasize)  // Copy incoming calldata
             let result := delegatecall(gas, implementation, ptr, calldatasize, 0, 0)
             let size := returndatasize
             returndatacopy(ptr, 0, size)

             switch result
             case 0 { revert(ptr, size) }
             default { return(ptr, size) }
         }
     }