[compiler-v2] Field selection after resource indexing not mutable

[wrwg]

Consider code as below:

    package fun deposit(account: address, coin: Coin) acquires Account {
        // What I have to write
        //(&mut Account[account].balance).merge(coin)
        // What I wish to write but got error message
        Account[account].balance.merge(coin)
    }

where fun merge(self: &mut Coin, Coin)

The produced error message is

error: cannot mutably borrow from an immutable ref
   ┌─ /Users/wrwg/move-demo/sources/account.move:20:9
   │
20 │         Account[account].balance.merge(coin)
   │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It appears that the decision to imm-ref the index Account[account] is taken before its known that it needs to be mutable. This might be a difficult problem to fix, but its a bit of annoying.