🔧 Add default configs for named versions

Config name can be :default, :current, :next, or :future.

lib/net/imap/config.rb

@@ -73,6 +73,10 @@ class IMAP
     #   client.config.sasl_ir                  # => true
     #   client.config.responses_without_block  # => :warn
     #
+    #   client = Net::IMAP.new(hostname, config: :future)
+    #   client.config.sasl_ir                  # => true
+    #   client.config.responses_without_block  # => :raise
+    #
     # The versioned default configs inherit certain specific config options from
     # Config.global, for example #debug:
     #
@@ -83,6 +87,26 @@ class IMAP
     #   Net::IMAP.debug = true
     #   client.config.debug?  # => true
     #
+    # === Named defaults
+    # In addition to +x.y+ version numbers, the following aliases are supported:
+    #
+    # [+:default+]
+    #   An alias for +:current+.
+    #
+    #   >>>
+    #   *NOTE*: This is _not_ the same as Config.default.  It inherits some
+    #   attributes from Config.global, for example: #debug.
+    # [+:current+]
+    #   An alias for the current +x.y+ version's defaults.
+    # [+:next+]
+    #   The _planned_ config for the next +x.y+ version.
+    # [+:future+]
+    #   The _planned_ eventual config for some future +x.y+ version.
+    #
+    # For example, to raise exceptions for all current deprecations:
+    #   client = Net::IMAP.new(hostname, config: :future)
+    #   client.responses  # raises an ArgumentError
+    #
     # == Thread Safety
     #
     # *NOTE:* Updates to config objects are not synchronized for thread-safety.
@@ -128,6 +152,8 @@ def self.[](config)
             case config
             when Numeric
               raise RangeError, "unknown config version: %p" % [config]
+            when Symbol
+              raise KeyError, "unknown config name: %p" % [config]
             else
               raise TypeError, "no implicit conversion of %s to %s" % [
                 config.class, Config
@@ -275,6 +301,14 @@ def to_h; data.members.to_h { [_1, send(_1)] } end
         responses_without_block: :warn,
       ).freeze
 
+      version_defaults[:default] = Config[0.4]
+      version_defaults[:current] = Config[0.4]
+      version_defaults[:next]    = Config[0.5]
+
+      version_defaults[:future]  = Config[0.5].dup.update(
+        responses_without_block: :raise,
+      ).freeze
+
       version_defaults.freeze
     end
   end

test/net/imap/test_config.rb

@@ -164,6 +164,17 @@ class ConfigTest < Test::Unit::TestCase
     assert_raise(RangeError) do Config[1] end
   end
 
+  test ".[] key errors" do
+    assert_raise(KeyError) do Config[:nonexistent] end
+  end
+
+  test ".[] with symbol names" do
+    assert_same    Config[0.4],   Config[:current]
+    assert_same    Config[0.4],   Config[:default]
+    assert_same    Config[0.5],   Config[:next]
+    assert_kind_of Config,        Config[:future]
+  end
+
   test ".[] with a hash" do
     config = Config[{responses_without_block: :raise, sasl_ir: false}]
     assert config.frozen?
@@ -179,6 +190,7 @@ class ConfigTest < Test::Unit::TestCase
     assert_same Config.default, Config.new(Config.default).parent
     assert_same Config.global,  Config.new(Config.global).parent
     assert_same Config[0.4],    Config.new(0.4).parent
+    assert_same Config[0.5],    Config.new(:next).parent
     assert_equal true, Config.new({debug: true}, debug: false).parent.debug?
     assert_equal true, Config.new({debug: true}, debug: false).parent.frozen?
   end