JSON is still an adapter specific type.

Several changes were made in #21110 which I am strongly opposed to.
(this is what I get for going on vacation. :trollface:) No type should
be introduced into the generic `ActiveRecord::Type` namespace, and
*certainly* should not be registered into the registry unconstrained
unless it is supported by *all* adapters (which basically means that it
was specified in the ANSI SQL standard).

I do not think `# :nodoc:` ing the type is sufficient, as it still makes
the code of Rails itself very unclear as to what the role of that class
is. While I would argue that this shouldn't even be a super class, and
that MySql and PG's JSON types are only superficially duplicated (they
might look the same but will change for different reasons in the
future).

However, I don't feel strongly enough about it as a point of contention
(and the biggest cost of harming the blameability has already occured),
so I simply moved the superclass into a namespace where its role is
absolutely clear.

After this change, `attribute :foo, :json` will once again work with
MySQL and PG, but not with Sqlite3 or any third party adapters.

Unresolved questions
--------------------

The types that and adapter publishes (at least those are unique to that
adapter, and not adding additional behavior like `MysqlString` should
probably be part of the adapter's public API. Should we standardize the
namespace for these, and document them?

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -1050,7 +1050,7 @@ module ActiveRecord
         end
       end
 
-      class MysqlJson < Type::Json # :nodoc:
+      class MysqlJson < Type::Internal::AbstractJson # :nodoc:
         def changed_in_place?(raw_old_value, new_value)
           # Normalization is required because MySQL JSON data format includes
           # the space between the elements.

activerecord/lib/active_record/connection_adapters/postgresql/oid.rb

@@ -8,6 +8,7 @@ require 'active_record/connection_adapters/postgresql/oid/decimal'
 require 'active_record/connection_adapters/postgresql/oid/enum'
 require 'active_record/connection_adapters/postgresql/oid/hstore'
 require 'active_record/connection_adapters/postgresql/oid/inet'
+require 'active_record/connection_adapters/postgresql/oid/json'
 require 'active_record/connection_adapters/postgresql/oid/jsonb'
 require 'active_record/connection_adapters/postgresql/oid/money'
 require 'active_record/connection_adapters/postgresql/oid/point'

activerecord/lib/active_record/connection_adapters/postgresql/oid/json.rb

@@ -0,0 +1,10 @@
+module ActiveRecord
+  module ConnectionAdapters
+    module PostgreSQL
+      module OID # :nodoc:
+        class Json < Type::Internal::AbstractJson
+        end
+      end
+    end
+  end
+end

activerecord/lib/active_record/connection_adapters/postgresql/oid/jsonb.rb

@@ -2,7 +2,7 @@ module ActiveRecord
   module ConnectionAdapters
     module PostgreSQL
       module OID # :nodoc:
-        class Jsonb < Type::Json # :nodoc:
+        class Jsonb < Json # :nodoc:
           def type
             :jsonb
           end

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -482,7 +482,7 @@ module ActiveRecord
           m.register_type 'bytea', OID::Bytea.new
           m.register_type 'point', OID::Point.new
           m.register_type 'hstore', OID::Hstore.new
-          m.register_type 'json', Type::Json.new
+          m.register_type 'json', OID::Json.new
           m.register_type 'jsonb', OID::Jsonb.new
           m.register_type 'cidr', OID::Cidr.new
           m.register_type 'inet', OID::Inet.new
@@ -838,6 +838,7 @@ module ActiveRecord
         ActiveRecord::Type.register(:enum, OID::Enum, adapter: :postgresql)
         ActiveRecord::Type.register(:hstore, OID::Hstore, adapter: :postgresql)
         ActiveRecord::Type.register(:inet, OID::Inet, adapter: :postgresql)
+        ActiveRecord::Type.register(:json, OID::Json, adapter: :postgresql)
         ActiveRecord::Type.register(:jsonb, OID::Jsonb, adapter: :postgresql)
         ActiveRecord::Type.register(:money, OID::Money, adapter: :postgresql)
         ActiveRecord::Type.register(:point, OID::Point, adapter: :postgresql)

activerecord/lib/active_record/type.rb

@@ -10,7 +10,6 @@ require 'active_record/type/decimal'
 require 'active_record/type/decimal_without_scale'
 require 'active_record/type/float'
 require 'active_record/type/integer'
-require 'active_record/type/json'
 require 'active_record/type/serialized'
 require 'active_record/type/string'
 require 'active_record/type/text'
@@ -21,6 +20,8 @@ require 'active_record/type/adapter_specific_registry'
 require 'active_record/type/type_map'
 require 'active_record/type/hash_lookup_type_map'
 
+require 'active_record/type/internal/abstract_json'
+
 module ActiveRecord
   module Type
     @registry = AdapterSpecificRegistry.new
@@ -60,7 +61,6 @@ module ActiveRecord
     register(:decimal, Type::Decimal, override: false)
     register(:float, Type::Float, override: false)
     register(:integer, Type::Integer, override: false)
-    register(:json, Type::Json, override: false)
     register(:string, Type::String, override: false)
     register(:text, Type::Text, override: false)
     register(:time, Type::Time, override: false)

activerecord/lib/active_record/type/internal/abstract_json.rb

@@ -0,0 +1,33 @@
+module ActiveRecord
+  module Type
+    module Internal # :nodoc:
+      class AbstractJson < Type::Value # :nodoc:
+        include Type::Helpers::Mutable
+
+        def type
+          :json
+        end
+
+        def deserialize(value)
+          if value.is_a?(::String)
+            ::ActiveSupport::JSON.decode(value) rescue nil
+          else
+            value
+          end
+        end
+
+        def serialize(value)
+          if value.is_a?(::Array) || value.is_a?(::Hash)
+            ::ActiveSupport::JSON.encode(value)
+          else
+            value
+          end
+        end
+
+        def accessor
+          ActiveRecord::Store::StringKeyedHashAccessor
+        end
+      end
+    end
+  end
+end

activerecord/lib/active_record/type/json.rb

@@ -1,31 +0,0 @@
-module ActiveRecord
-  module Type
-    class Json < Type::Value # :nodoc:
-      include Type::Helpers::Mutable
-
-      def type
-        :json
-      end
-
-      def deserialize(value)
-        if value.is_a?(::String)
-          ::ActiveSupport::JSON.decode(value) rescue nil
-        else
-          value
-        end
-      end
-
-      def serialize(value)
-        if value.is_a?(::Array) || value.is_a?(::Hash)
-          ::ActiveSupport::JSON.encode(value)
-        else
-          value
-        end
-      end
-
-      def accessor
-        ActiveRecord::Store::StringKeyedHashAccessor
-      end
-    end
-  end
-end