We don't really want to introduce new \textits. Consult with @jensmaurer for details.

Every other occurrence of meow-expr in this section uses \textit, which is why I used them here.

I understand that. We still need to talk about this.

It seems to me begin-expr (and its friend end-expr) should be \placeholder, actually: It's a meta-variable (for a chunk of code, in this case).

So \placeholder it throughout? That was admittedly my first thought, before seeing all-\textits in the surroundings.

See #1558.

This should be \defn{explicitly-defaulted definition}, probably with "definition!explicitly-defaulted" as the main index entry and a "see" pointer for "explicitly-defaulted definition".

There's already a \indextext{definition!function!explicitly-defaulted} ~10 lines above, which is why I went with \term.

In my opinion, \term should only (if at all) survive for stuff that is local enough not to ever be referenced from elsewhere, and thus not needing an index entry at all. For this particular case, it seems the \indextext from before should simply be folded into the \defnx to be used after "is called an".

I'm slightly concerned that we seem to define "explicitly-defaulted definition" as the technical term, although we mean "explicitly-defaulted function definition". That probably needs a bit of research what we actually want here.