How to perform delegate call inside of view call (staticall)?

Question

Inside view method it's not possible to do delegatecall due to compilation errors. Is there any way to delegate view calls?

Answer 1

UPDATE: Check out my new answer with 2 different solutions to the similar issue here: https://ethereum.stackexchange.com/a/142712/3032

Yep, but it is tricky:

pragma solidity ^0.5.0;

contract D {
    function g(uint256 a) public view returns(uint256) {
        return a*2;
    }
}

contract A {
    D public impl;
    
    constructor() public {
        impl = new D();
    }
    
    function() external {
        require(msg.sender == address(this));
        
        (bool success, bytes memory data) = address(impl).delegatecall(msg.data);
        assembly {
            switch success
                // delegatecall returns 0 on error.
                case 0 { revert(add(data, 32), returndatasize) }
                default { return(add(data, 32), returndatasize) }
        }
    }
    
    function f(uint256 a)
        public
        view
        returns(uint256)
    {
        (bool success, bytes memory data) = address(this).staticcall(
            abi.encodeWithSelector(
                impl.g.selector,
                10
            )
        );
        
        assembly {
            switch success
                // delegatecall returns 0 on error.
                case 0 { revert(add(data, 32), returndatasize) }
                default { return(add(data, 32), returndatasize) }
        }
    }
}