[Ruby] 3.15 regression: Cannot get repeated field values using bracket notation

[dazuma]

What version of protobuf and what language are you using?
Version: 3.15.0
Language: Ruby

What operating system (Linux, Windows, ...) and version?
Various (tried on both gLinux x86_64, Mac Catalina x86_64)

What runtime / compiler are you using (e.g., python version or gcc version)
Using the prebuilt release binary for Ruby.

What did you do?

require "google/protobuf"
Google::Protobuf::DescriptorPool.generated_pool.build do
  add_file("blah.proto", syntax: :proto3) do
    add_message "TestProto" do
      optional :single, :string, 1
      repeated :multi, :string, 2
    end
  end
end
TestProto = Google::Protobuf::DescriptorPool.generated_pool.lookup("TestProto").msgclass
message = TestProto.new(single: "hello", multi: ["a", "b"])
p message.single
p message["single"]
p message.multi
p message["multi"]

What did you expect to see
This is what I see under google-protobuf 3.14.0:

"hello"
"hello"
["a", "b"]
["a", "b"]

What did you see instead?
This is what I see under google-protobuf 3.15.0:

"hello"
"hello"
["a", "b"]
""

[haberman]

Closing as duplicate of #8311