Since Firstname is a string literal defined in the function scope, missing to explicitly define the data location sets it to storage by default (where the state variables are persisted) [1]. That assignment operation then looks for the data at the provided location, which in the example is not a valid storage reference.
In order to solve the compilation error in the provided example, it is necessary to set the memory keyword, as pointed out by user smarx: string memory Firstname = "Hello";.
The second snippet is valid, as of Solidity version 0.4.25.
Given the provided code snippets, the following would be the aggregated code, to serve as reference (SecondContract.sol):
pragma solidity ^0.4.25;
contract UserRegistration {
function getUserData(address userAddress) public constant returns(string, uint) {
string memory Firstname = "Hello";
uint number=123;
return( Firstname,number);
}
}
contract SecondContract {
function FetchDetail(address userAddress) public constant returns(string,uint ) {
string memory data1;
uint data2;
UserRegistration ud=new UserRegistration();
(data1,data2)= ud.getUserData(userAddress);
return(data1,data2);
}
}
Deploying SecondContract and calling FetchDetail function with 0x0000000000000000000000000000000000000000 as argument returns:
uint256 123
[1] https://solidity.readthedocs.io/en/v0.4.25/types.html#data-location
Stay super,!
Javier/Javi
