Mutation methods type-locking question

We (at Datadog) stumble upon situation when we use Hash#transform_values! and we get into the trap of type-locking introduced in this PR https://github.com/ruby/rbs/pull/728

What do you think of making both mutation methods have the same signature, like this one (taken from the original transform_values)?

  def transform_values: () -> Enumerator[V, Hash[K, untyped]]
                      | [A] () { (V) -> A } -> Hash[K, A]

Because now there is no way to change the type of say, this expression (response is Net::HTTPResponse)

response.to_hash.transform_values! { |value| Array(value).join }

when run Steep it will complain that original Hash typed as Hash[String, Array[String]] and if something expects to get Hash[String, String] the check will fail, but in fact we mutate the value to be a String.

Start with the Hash#transform_values! signature described in this issue and compare it with the original transform_values signature from ruby/rbs#728. Reproduce the Net::HTTPResponse example with Steep and check whether the mutation can change Hash[String, Array[String]] to Hash[String, String] without a type error.