fallback() should allow returns

[3esmit]

Abstract

Allow a fallback function to use returns.

Motivation

Some YUL programs use return inside fallback, e.g. https://github.com/Zoltu/deterministic-deployment-proxy/blob/49f29698ce95bd510a18cabb94fd8ba4d352d687/source/deterministic-deployment-proxy.yul#L15

I can't easily adapt that behavior in solidity, this is how I worked around it (using return opcode):

    fallback() external payable {
        address payable createdContract = deploy(msg.data, bytes32(0));
        assembly {
            mstore(0, createdContract)
            return(0, 32)
        }
    }
   function deploy(bytes,bytes32){ /**create2**/ }

Would be good if I could just return the type I want there without using assembly.

Specification

if function is fallback, the call of return could be anything, because it would be directly a opcode call of return, which would be a bytes output.

    fallback() external payable returns(bytes memory returnData) {
        address payable createdContract = deploy(msg.data, bytes32(0));
        returnData = abi.encode(createdContract);
    }

Also for contracts that always return the same data type at fallback, they could use returns like any other function:

    fallback() external payable returns(address payable createdContract) {
        createdContract = deploy(msg.data, bytes32(0));
    }

This would not work for receive(), only for fallback.

Backwards Compatibility

N/A

[chriseth]

I don't see why we should not support this.

[leonardoalt]

Sounds good to me as well.

[axic]

We had a very long discussion about the topic in #3198.

[chriseth]

In the meeting @axic mentioned that it should be fallback(bytes calldata input) external payable returns (bytes memory output) {}

[axic]

Also relevant: #2630