Properly lookup the limit for bigint

Fixes #18787.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -453,7 +453,12 @@ module ActiveRecord
       end
 
       def extract_limit(sql_type) # :nodoc:
-        $1.to_i if sql_type =~ /\((.*)\)/
+        case sql_type
+        when /^bigint/i
+          8
+        when /\((.*)\)/
+          $1.to_i
+        end
       end
 
       def translate_exception_class(e, sql)

activerecord/test/cases/connection_adapters/type_lookup_test.rb

@@ -79,6 +79,11 @@ module ActiveRecord
         assert_lookup_type :integer, 'bigint'
       end
 
+      def test_bigint_limit
+        cast_type = @connection.type_map.lookup("bigint")
+        assert_equal 8, cast_type.limit
+      end
+
       def test_decimal_without_scale
         types = %w{decimal(2) decimal(2,0) numeric(2) numeric(2,0) number(2) number(2,0)}
         types.each do |type|