[Ethereum] How to pass a blank bytes calldata into a solidity function call

You are getting this error because you are trying to take the keccack256 hash of a uint, which you cannot do, according to the docs. What you must do instead is convert the uint to a bytes.

The code you want is as follows:

function random() private returns(uint){
    uint source = block.difficulty + now;
    bytes memory source_b = toBytes(source);
    return uint(keccak256(source_b));
}

function toBytes(uint256 x) public returns (bytes memory b) {
    b = new bytes(32);
    assembly { mstore(add(b, 32), x) }
}

When you declare a parameter as storage you are limiting the function to only accept parameters with that qualifier.

The qualifiers storage, memory and calldata specify where the data is located. You can convert implicitly storage to memory and calldata to memory, but the reverse is not possible.

string public stor = "banana";

function test(string calldata calld) external view {
    string memory memo = "pizza";

    foo(memo, stor);
    foo(calld, stor);  // Creates a copy of calld in memory and passes as parameter
    foo(stor, stor);   // Creates a copy of stor in memory and passes as parameter
    
    foo(memo, stor);
    // foo(memo, memo); // Cannot convert from memory to storage
    // foo(memo, calld); // Cannot convert from calldata to storage
}

function foo(string memory, string storage) internal view {
    
}

/* This fails because you can only use calldata on external functions
function bar(string calldata) internal view {
    
} */

Value types like bool, uintXXX, bytesYY, address, etc. are passed by value. Non-value tyeps like structs, arrays, string are passed by reference.

• Functions that are public, internal and private can use types passed by reference but only if they are call from the same contract.

• Functions public and external will receive a copy when called by a regular user of from another contract.

I don't know why, but at least since solc 0.3 it wasn't allowed.