Solidity – Can Internal Functions Be Called Using delegatecall?

delegatecall is not supposed to handle return values as it cannot be aware of the length of the other side (unless there would be a dedicated parameter passed to it). Currently, the delegatecall Solidity built-in method does not support supplying the return value size, nor getting the return value out after the call.

According to the documentation it should always return a boolean signaling the execution outcome. It seems the compiler may currently have a problem, see https://github.com/ethereum/solidity/issues/2678.

To get the result out, you will need to know the return value size and use inline assembly:

assembly {
    let returnSize = 32
    calldatacopy(0xff, 0, calldatasize)
    let _retVal = delegatecall(gas, currentVersion, 0xff, calldatasize, 0, returnSize)
    switch _retval case 0 { revert(0,0) } default { return(0, returnSize) }
}

The issue is that the response variable is not a memory pointer, it's a stack variable. When you use it as the pointer for your out data in the delegatecall, you're actually just writing the data to the 0 memory location. This doesn't cause errors because that space is reserved for "scratch space", but it also never loads the result into your return variable.

What you want to do is

assembly {
        let status := delegatecall(sub(gas,5000), dest, add(calldata, 0x20), mload(calldata), 0, 32)
        response := mload(0)
        jumpi(invalidJumpLabel, iszero(status))
}