TypeError: Library functions must be implemented if declared for solc compiler version ^0.6.0 library implementation

Question

Suppose I have the following code:

// SPDX-License-Identifier: MIT
pragma solidity >=0.4.21 <0.7.0;

library SomeLib {
    function add(uint256 a, uint256 b) public returns(uint256);
}

contract SomeContract {
    
    function calc(uint256 a, uint256 b) public returns(uint256) {
        uint256 ans = SomeLib.add(a, b);
        return ans;
    }
}

It works with no problem if my solc compiler version < 0.6.0 however, it fails to compile version >= 0.6.0 returns following:

TypeError: Library functions must be implemented if declared.

Any idea or something missing here?

Here are the setup:

$ npx truffle version
Truffle v5.1.56 (core: 5.1.56)
Solidity - 0.6.12 (solc-js)
Node v12.18.3
Web3.js v1.2.9