solidity/test/libsolidity/semanticTests/interface_inheritance_conversions.sol

interface Parent {
    function parentFun() external returns (uint256);
}

interface SubA is Parent {
    function subAFun() external returns (uint256);
}

interface SubB is Parent {
    function subBFun() external returns (uint256);
}

contract Impl is SubA, SubB {
    function parentFun() override external returns (uint256) { return 1; }
    function subAFun() override external returns (uint256) { return 2; }
    function subBFun() override external returns (uint256) { return 3; }
}

contract C {
    function convertParent() public returns (uint256) {
        Parent p = new Impl();
        return p.parentFun();
    }

    function convertSubA() public returns (uint256, uint256) {
        SubA sa = new Impl();
        return (sa.parentFun(), sa.subAFun());
    }

    function convertSubB() public returns (uint256, uint256) {
        SubB sb = new Impl();
        return (sb.parentFun(), sb.subBFun());
    }
}

// ----
// convertParent() -> 1
// convertSubA() -> 1, 2
// convertSubB() -> 1, 3