Example to show how `contract` type can be used when defining `constant` or `immutable`

ContractImmutable.sol

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract ContractTemplate {
    // some logic here...
}

abstract contract MyContract {

    // This will not compile and throw the error
    // TypeError: Initial value for constant variable has to be compile-time constant.
    ContractTemplate constant a = new ContractTemplate();
    
    ContractTemplate immutable b = new ContractTemplate();
    ContractTemplate immutable c;

    constructor() {
        c = new ContractTemplate();
    }

}