Add tests for `TypeMap#fetch` and push up to `TypeMap`

It doesn't make sense for the subclass to implement this method, and not
have it on the parent. We can also DRY up the implementation of
`#lookup` to be defined in terms of fetch, which will give us a single
point of entry

activerecord/lib/active_record/type/hash_lookup_type_map.rb

@@ -3,10 +3,6 @@ module Type
     class HashLookupTypeMap < TypeMap # :nodoc:
       delegate :key?, to: :@mapping
 
-      def lookup(type, *args)
-        @mapping.fetch(type, proc { default_value }).call(type, *args)
-      end
-
       def fetch(type, *args, &block)
         @mapping.fetch(type, block).call(type, *args)
       end

activerecord/lib/active_record/type/type_map.rb

@@ -6,6 +6,10 @@ def initialize
       end
 
       def lookup(lookup_key, *args)
+        fetch(lookup_key, *args) { default_value }
+      end
+
+      def fetch(lookup_key, *args)
         matching_pair = @mapping.reverse_each.detect do |key, _|
           key === lookup_key
         end
@@ -13,7 +17,7 @@ def lookup(lookup_key, *args)
         if matching_pair
           matching_pair.last.call(lookup_key, *args)
         else
-          default_value
+          yield lookup_key, *args
         end
       end
 

activerecord/test/cases/type/type_map_test.rb

@@ -124,6 +124,21 @@ def test_lookup_non_strings
         assert_equal mapping.lookup(3), 'string'
         assert_kind_of Type::Value, mapping.lookup(4)
       end
+
+      def test_fetch
+        mapping = TypeMap.new
+        mapping.register_type(1, "string")
+
+        assert_equal "string", mapping.fetch(1) { "int" }
+        assert_equal "int", mapping.fetch(2) { "int" }
+      end
+
+      def test_fetch_yields_args
+        mapping = TypeMap.new
+
+        assert_equal "foo-1-2-3", mapping.fetch("foo", 1, 2, 3) { |*args| args.join("-") }
+        assert_equal "bar-1-2-3", mapping.fetch("bar", 1, 2, 3) { |*args| args.join("-") }
+      end
     end
   end
 end