```ruby
class Post < ActiveRecord::Base
  attribute :written_at, default: -> { Time.now.utc }
end

# Rails 6.0
Post.type_for_attribute(:written_at) # => #<Type::Value ... precision: nil, ...>

# Rails 6.1
Post.type_for_attribute(:written_at) # => #<Type::DateTime ... precision: 6, ...>
```

This is an alternative of #39797.

Context https://github.com/rails/rails/pull/39797#issuecomment-655191817.

If people intend to override the existing type on the attribute, usually
an overriding type is explicitly specified, so I agree that the current
behavior (drop existing type information if type is omitted) is
practically quite useless, and it is almost like a bug in that there is
no way to override just the default.

So I'd like to change the current behavior on existing attributes
without a deprecation as a fix.

Closes #39797.

See also https://github.com/kufu/activerecord-bitemporal/pull/57.