[concepts.callable.general] p1

The concepts in 18.7 describe the requirements on function objects…

[concept.invocable] p1

The invocable concept specifies a relationship between a callable type…

"Function objects" and "callable type" does not include important cases covered by INVOKE, such as references to functions. "Callable types, function types, and references to functions" (or just a "type") would be better.

Since these are introductory paragraphs, I think this is an editorial issue.