✨ Responses with type can return frozen dup array

Because responses(type) is relatively new and has always raised an
exception, we can update it to return a frozen dup array without
breaking backward compatibility.

Additionally, `config.responses_without_args` was added as an alias for
`config.responses_without_block`.  The original name is a little
misleading now, but it's kept for backward compatibility.

lib/net/imap.rb

@@ -2503,17 +2503,21 @@ def idle_done
     private_constant :RESPONSES_DEPRECATION_MSG
 
     # :call-seq:
+    #   responses(type) -> frozen array
     #   responses       {|hash|  ...} -> block result
     #   responses(type) {|array| ...} -> block result
     #
-    # Yields unhandled server responses and returns the result of the block.
-    # Unhandled responses are stored in a hash, with arrays of
-    # <em>non-+nil+</em> UntaggedResponse#data keyed by UntaggedResponse#name
-    # and ResponseCode#data keyed by ResponseCode#name.
+    # Yields or returns unhandled server responses.  Unhandled responses are
+    # stored in a hash, with arrays of UntaggedResponse#data keyed by
+    # UntaggedResponse#name and <em>non-+nil+</em> untagged ResponseCode#data
+    # keyed by ResponseCode#name.
+    #
+    # When a block is given, yields unhandled responses and returns the block's
+    # result.  Without a block, returns the unhandled responses.
     #
     # [With +type+]
-    #   Yield only the array of responses for that +type+.
-    #   When no block is given, an +ArgumentError+ is raised.
+    #   Yield or return only the array of responses for that +type+.
+    #   When no block is given, the returned array is a frozen copy.
     # [Without +type+]
     #   Yield or return the entire responses hash.
     #
@@ -2534,7 +2538,7 @@ def idle_done
     # For example:
     #
     #   imap.select("inbox")
-    #   p imap.responses("EXISTS", &:last)
+    #   p imap.responses("EXISTS").last
     #   #=> 2
     #   p imap.responses("UIDNEXT", &:last)
     #   #=> 123456
@@ -2592,7 +2596,7 @@ def responses(type = nil)
       if block_given?
         synchronize { yield(type ? @responses[type.to_s.upcase] : @responses) }
       elsif type
-        raise ArgumentError, "Pass a block or use #clear_responses"
+        synchronize { @responses[type.to_s.upcase].dup.freeze }
       else
         case config.responses_without_block
         when :raise

lib/net/imap/config.rb

@@ -240,10 +240,19 @@ def self.[](config)
       # [+:raise+ <em>(planned future default)</em>]
       #   Raise an ArgumentError with the deprecation warning.
       #
+      # Note: #responses_without_args is an alias for #responses_without_block.
       attr_accessor :responses_without_block, type: [
-        :silence_deprecation_warning, :warn, :raise,
+        :silence_deprecation_warning, :warn, :frozen_dup, :raise,
       ]
 
+      alias responses_without_args  responses_without_block  # :nodoc:
+      alias responses_without_args= responses_without_block= # :nodoc:
+
+      ##
+      # :attr_accessor: responses_without_args
+      #
+      # Alias for responses_without_block
+
       # Creates a new config object and initialize its attribute with +attrs+.
       #
       # If +parent+ is not given, the global config is used by default.

test/net/imap/test_imap_responses.rb

@@ -89,12 +89,35 @@ def for_each_config_option(imap)
     end
   end
 
-  # with with a type and no block: always raise an exception
+  # with with a type and no block: always returns a frozen duplicate
   test "#responses(type, &nil)" do
     with_fake_server do |server, imap|
-      for_each_config_option(imap) do
-        assert_raise(ArgumentError) do imap.responses("CAPABILITY") end
+      stderr = EnvUtil.verbose_warning do
+        # Config options make no difference to responses(type)
+        for_each_config_option(imap) do
+          # responses available before SELECT/EXAMINE
+          assert imap.responses("CAPABILITY").frozen?
+          assert_equal(%w[IMAP4REV1 NAMESPACE MOVE IDLE UTF8=ACCEPT],
+                       imap.responses("CAPABILITY").last)
+        end
+        # responses are cleared after SELECT/EXAMINE
+        imap.select "INBOX"
+        for_each_config_option(imap) do
+          assert imap.responses("CAPABILITY").frozen?
+          assert imap.responses("EXISTS").frozen?
+          assert imap.responses("UIDVALIDITIY").frozen?
+          assert_equal [],           imap.responses("CAPABILITY")
+          assert_equal [172],        imap.responses("EXISTS")
+          assert_equal [3857529045], imap.responses("UIDVALIDITY")
+          assert_equal 1, imap.responses("RECENT").last
+          assert imap.responses("UIDNEXT").frozen?
+          assert_equal [4392], imap.responses("UIDNEXT")
+          assert imap.responses("FLAGS").frozen?
+          assert_equal(%i[Answered Flagged Deleted Seen Draft],
+                       imap.responses("FLAGS").last)
+        end
       end
+      assert_empty stderr # never warn when type is given
     end
   end