tl;dr You have a syntax error. Use T::Array[Object] (not Array[Object]).

You're using incorrect type syntax for arrays:

# typed: strict

extend T::Sig

sig { params(foo: Array[Object]).returns(Array[Object]) }
def bar(foo)
  # code to modify some of the attributes
end

→ View on sorbet.run

The errors show:

editor.rb:5: Use T::Array[...], not Array[...] to declare a typed Array https://srb.help/5026
     5 |sig { params(foo: Array[Object]).returns(Array[Object]) }
                                                 ^^^^^^^^^^^^^
  Note:
    Array[...] will raise at runtime because this generic was defined in the standard library
  Autocorrect: Use `-a` to autocorrect
    editor.rb:5: Replace with T::Array
     5 |sig { params(foo: Array[Object]).returns(Array[Object]) }
                                                 ^^^^^

editor.rb:5: Use T::Array[...], not Array[...] to declare a typed Array https://srb.help/5026
     5 |sig { params(foo: Array[Object]).returns(Array[Object]) }
                          ^^^^^^^^^^^^^
  Note:
    Array[...] will raise at runtime because this generic was defined in the standard library
  Autocorrect: Use `-a` to autocorrect
    editor.rb:5: Replace with T::Array
     5 |sig { params(foo: Array[Object]).returns(Array[Object]) }
                          ^^^^^
Errors: 2

Why is it like this?

The [] method on Array has special meaning, but Sorbet uses [] for generic type arguments. To get around that, Sorbet uses the T:: namespace for certain generic classes in the standard library:

→ https://sorbet.org/docs/stdlib-generics

What's happening in your case is that this code:

Array[Object]

is equivalent to having written this:

[Object]

(i.e., "make an array of length one containing the value Object"). [Object] in Sorbet happens to be the way to express a 1-tuple.