At the moment, closed type families without any equations fail with a parse error. In addition, they cannot be created by TH (see #8028 (closed)). Would it be possible to permit these instead?

My use case is in my typechecker plugin for units of measure, where I want to add new type-level operators without any equational theory (because it will be supplied by the plugin) and without users having the ability to introduce their own type family instances.