Errors raised in `type_cast_for_database` no longer raise on assignment

Fixes #18580.

activerecord/CHANGELOG.md

+*   Values which would error while being sent to the database (such as an
+    ASCII-8BIT string with invalid UTF-8 bytes on Sqlite3), no longer error on
+    assignment. They will still error when sent to the database, but you are
+    given the ability to re-assign it to a valid value.
+
+    Fixes #18580.
+
+    *Sean Griffin*
+
 *   Don't remove join dependencies in `Relation#exists?`
 
-    Fixes #18632
+    Fixes #18632.
 
     *Sean Griffin*
 

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -165,7 +165,7 @@ def original_raw_attributes
       end
 
       def store_original_raw_attribute(attr_name)
-        original_raw_attributes[attr_name] = @attributes[attr_name].value_for_database
+        original_raw_attributes[attr_name] = @attributes[attr_name].value_for_database rescue nil
       end
 
       def store_original_raw_attributes

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -53,7 +53,7 @@ def cast_value(value)
     class SQLite3String < Type::String # :nodoc:
       def type_cast_for_database(value)
         if value.is_a?(::String) && value.encoding == Encoding::ASCII_8BIT
-          value.encode(Encoding::UTF_8)
+          value.encode(Encoding::UTF_8, undef: :replace)
         else
           super
         end

activerecord/test/cases/types_test.rb

@@ -117,6 +117,23 @@ def test_binary_encoding
           assert_equal Encoding::ASCII_8BIT, type_cast.encoding
         end
       end
+
+      def test_attributes_which_are_invalid_for_database_can_still_be_reassigned
+        type_which_cannot_go_to_the_database = Type::Value.new
+        def type_which_cannot_go_to_the_database.type_cast_for_database(*)
+          raise
+        end
+        klass = Class.new(ActiveRecord::Base) do
+          self.table_name = 'posts'
+          attribute :foo, type_which_cannot_go_to_the_database
+        end
+        model = klass.new
+
+        model.foo = "foo"
+        model.foo = "bar"
+
+        assert_equal "bar", model.foo
+      end
     end
   end
 end