Also related, is in upgrading proxies - Storage collisions cause non-trivial errors. Obviously, it's harder to verify this because an upgrade could happen at a distant point in the future.

Old implementation:

contract Foo { 
  uint public b = 5;
};

New implementation:

// Incorrect
contract Bar { 
  uint public c; // Collision here! - value is actually set to 5 not 0
}

// Correct
contract Bar {
  uint public b; // value is 5 as expected
  uint public c; // value is 0
}

It would be useful to have a function that checks that the new contract doesn't conflict with the old contracts storage and is a valid proxy upgrade.

// Checks if "Foo" can be upgraded to "Bar"
const isValid = deployments.isValidProxyUpgrade("Foo", "Bar");

OpenZeppelin has a similar check but it is in-built into their deploy function so is of no use here in hardhat-deploy although their verification code can probably be mostly reused (?)

Definitely wanted :), this was also mentioned here : #65

closing as duplicate of #65