[Bug][Compiler-v2] V2 does not generate the error mutable ownership violated

[rahxephon89]

🐛 Bug

For the following code:

module 0x8675309::A {
    use std::signer;
    struct T has key {v: u64}

    public fun t0(account: &signer) acquires T {
        let sender = signer::address_of(account);
        let x = borrow_global_mut<T>(sender);
        copy x;
        x = borrow_global_mut<T>(sender);
        copy x;
    }
}

V1 generates the error:

error[E07002]: mutable ownership violated
  ┌─ tests/move_check/translated_ir_tests/move/borrow_tests/borrow_global_bad0.move:9:13
  │
7 │         let x = borrow_global_mut<T>(sender);
  │                 ---------------------------- It is still being mutably borrowed by this reference
8 │         copy x;
9 │         x = borrow_global_mut<T>(sender);
  │             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Invalid borrowing of resource 'T'

While V2 does not generate any errors.

[rahxephon89]

This example V1 is overly restricted because copy x is not assigned and used after the second borrow global.

Moreover, for the following program:

module 0x8675309::A {
    use std::signer;
    struct T has key {v: u64}

    public fun t0(account: &signer) acquires T {
        let sender = signer::address_of(account);
        let x = borrow_global_mut<T>(sender);
        let y = copy x;
        x = borrow_global_mut<T>(sender);
        copy y;
    }
}

V2 can correctly generate the following error:

Diagnostics:
error: cannot mutably borrow since mutable references exist
   ┌─ tests/reference-safety/debug-13148.move:9:13
   │
 7 │         let x = borrow_global_mut<T>(sender);
   │                 ---------------------------- previous mutable global borrow
 8 │         let y = copy x;
 9 │         x = borrow_global_mut<T>(sender);
   │         ----^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   │         │   │
   │         │   mutable borrow attempted here
   │         requirement enforced here
10 │         copy y;
   │         ------ conflicting reference `y` used here

IMHO, we can close this issue? @wrwg