Does "this" keyword changes msg.sender on inherited methods invocations?

Question

Take this scenario:

Contract A {
   function whoIsSender() public returns (address){
      return (msg.sender);
   }
}

Contract B is A {
   function withThis() public returns (address){
      return (this.whoIsSender());
   }

   function withoutThis() public returns (address){
      return (whoIsSender());
   }
}

I was expecting both functions to return the same. Apparently it is not. It seems that using the this keyword breaks it to a contract-to-contract invocation and then msg.sender becomes the contract address instead of the wallet? Is this by design and can someone explain why?

Answer 1

This is touched on a bit here: https://solidity.readthedocs.io/en/v0.4.21/contracts.html#visibility-and-getters, but essentially what you're saying is correct.

<address>.<function>() (e.g. this.whoIsSender()) sends a message to the contract at that address. This is an external call, and msg.sender is the address of the account that made the call.

<function>() (e.g. whoIsSender()) does an internal function call (just a JUMP).