Please check the existing index entries and find a suitable place. This one isn't.
Maybe "operator function!comparison!implicitly-defined".

For all my index entries, I do scan the existing ones and to try and find a suitable place. As I said in my "Notes/issues", it isn't clear how we should write index entries. I don't like having so many levels either, but which part to leave out, and which parts to combine?

That said, I see we already have existing definitions for "implicitly defined", none of which are top-level, and appear to use "implicitly defined" and "implicitly-defined" interchangeably throughout the wording. In [class.copy.ctor]/p12 for example, we define "implicitly defined" under copy/move constructors, then use "implicitly-defined" in the very same paragraph. I would like to a see top-level entry for "implicitly defined" which refers to these other cases in a second level, and to clean up the confusion between "implicitly defined" and "implicitly-defined" .

For this case, we don't have "operator function" but we do have "operator", so I'll just remove the "defaulted" level and remove the top level definition to be consistent with the other definitions. Thanks!

drop the \idxcode{@}; it doesn't help in the index entry.

This is how we format operators in the index, e.g.:

\indextext{operator!\idxcode{+=}}%
\indextext{\idxcode{++}|see{operator, increment}}%
\indextext{\idxcode{:}!label specifier}%
\indextext{\idxcode{for}!scope of declaration in}%

That said, the indexes for operators and keywords also needs a cleaned up. E.g. we have \indextext{\idxcode{for}!scope of declaration in}% but no top-level for, just \indextext{statement!\idxcode{for}}%.

drop the \indextext and just say \defnadj{secondary}{comparison operator}

I went with \defnadj{secondary}{operator!comparison} since we don't have a top level index entry for "comparison operator".

I don't think "!" renders correctly in the \defnadj. You need to do a manual \defnx if you want to get clever.