What does "caller()" return in Yul when DELEGATECALL is used?

Question

While browsing the Solidity docs, I noticed this instruction available in assembly:

caller(): call sender (excluding delegatecall)

What do the docs mean by "excluding delegatecall"? Is it that if contract A delegate calls to contract B, caller() will not return the address of contract A, but instead it would return the original EOA that called A? (I know that msg.sender is the same in this case)

Answer 1

It returns an empty number (0x000..0000).

You can check it yourself running the following code on Remix:

contract Stub {
  event Caller(uint caller);

  function testCaller() public {
      uint caller;
      assembly { caller := caller() }
      emit Caller(caller);
  }
}

contract Delegator {

  // where stub must be the address of the above contract.
  function testDelegatecall(address stub) public returns (bool) {
    (bool success,) = stub.delegatecall(
        abi.encodeWithSignature("testCaller()")
    );
    return success;
  }
}