Library calls fail depending on compiler version?

Question

I'm trying to deploy my library and call it afterwards with the specific address. While testing this I encountered that the behavior changed over the course of compilers.

I tried to follow How to call a library contract but the following inconsistency happened:

Library 1

pragma solidity 0.7.0;

library Seven {
    function seven() public returns (bool) {
        return true;
    }
}

Library 2

pragma solidity 0.4.6;

library Foursix {
    function foursix() public returns (bool) {
        return true;
    }
}

Main Contract

pragma solidity 0.7.0;
abstract contract Foursix {
    function foursix() virtual public returns (bool);
}

abstract contract Seven {
    function seven() virtual public returns (bool);
}

contract Foo {
    function callfoursix(address a) public {
        Foursix fs = Foursix(a);
        bool b = fs.foursix();
    }
    function callseven(address a) public {
        Seven s = Seven(a);
        bool b = s.seven();
    }
}

notice how Library 1 and 2 are identical besides the used compiler. When testing in remix Library 1 gets the error:

transact to Foo.callseven errored: VM error: revert.

while the call to Library 2 succeeds.

Why is this the case and how do I call the Library 1 contract? It does seem that ABI encoding has something to do with it as the newer version is seemingly not found but for the moment I'm clueless.

Answer 1

Turns out functions have to be declared as "view or pure" when called from outside after solidity version 0.5.

https://solidity.readthedocs.io/en/develop/contracts.html#call-protection-for-libraries