Incorrect success value returned in vyper

### Background During the audit of [Lido's Gate Seals](https://github.com/lidofinance/gate-seals) code [statemind](https://statemind.io) team identified a weird behavior of the code that uses `raw_call`: https://github.com/lidofinance/gate-seals/blob/051593e74df01a4131c485b4fda52e691cd4b7d8/contracts/GateSeal.vy#L164 . Construction like this: ```vyper success = raw_call( sealable, _abi_encode(SEAL_DURATION_SECONDS, method_id=method_id("pauseFor(uint256)")), revert_on_failure=False ) ``` was not fully documented: https://docs.vyperlang.org/en/v0.3.7/built-in-functions.html#raw_call . The documentation says that: if `max_outsize=0` it should return nothing and then it says that if `revert_on_failure=False` it should return a `success` flag in the tuple of response, but what if `max_outsize=0` and `revert_on_failure=False`. <img width="715" alt="image" src="https://user-images.githubusercontent.com/22330612/232125364-d2b3bbac-0b4f-40cb-80ff-f55d8eafef44.png"> So the team started researching what exactly happened in that case, after some research we found that the Vyper compiler generates the wrong bytecode in that case, it generates the sequence: ``` CALL // call MLOAD // MLOAD is wrong since the CALL result is already stored in the stack ``` ### Impact Example of buggy code: ```vyper @external def returnSome(calling: address, a: uint256) -> bool: success: bool = false success = raw_call( calling, _abi_encode(a, method_id=method_id("a(uint256)")), revert_on_failure=False ) ``` any contract that uses the `raw_call` with `revert_on_failure=False` and `max_outsize=0` receives the wrong response from `raw_call`. Depending on the memory garbage, the result can be either `True` or `False`. ### Patches Fix by @charles-cooper https://github.com/vyperlang/vyper/commit/851f7a1b3aa2a36fd041e3d0ed38f9355a58c8ae ### Workarounds The simple workaround is always to put `max_outsize>0`. Workaround example https://github.com/lidofinance/gate-seals/pull/5/files ### References Lido's fix: https://github.com/lidofinance/gate-seals/pull/5/files

Quoted source text, attributed separately from HOL analysis.