Subscript (index access)

[] is different from normal methods.

a = [!1, !2]
a[0].inc!()
assert a == [2, 2]

Recall that the return value of a subroutine cannot be a reference. The type of a[0] here should clearly be Ref!(Int!) (the type of a[0] depends on the context). So [] is actually part of a special syntax, just like .. Unlike Python, it cannot be overloaded. It is also not possible to reproduce the behavior of [] in a method.

C = Class {i = Int!}
C.steal(self) =
    self::i

C.get(ref self) =
    self::i # TypeError: `self::i` is `Int!` (require ownership) but `get` doesn't own `self`

# OK (assigning)
c = C.new({i = 1})
i = c.steal()
i.inc!()
assert i == 2
# or (own_do!)
own_do! C.new({i = 1}).steal(), i => i.inc!()

# NG
C.new({i = 1}).steal().inc!() # OwnershipWarning: `C.new({i = 1}).steal()` is not owned by anyone
# hint: assign to a variable or use `uwn_do!`

Also, [] can be disowned, but the element is not shifted.

a = [!1, !2]
i = a[0]
i.inc!()
assert a[1] == 2
a[0] # OwnershipError: `a[0]` is moved to `i`