[xindy] How to suppress 'Cross-reference-target (“entry”) does not exist' error for entries without locators

[Kevin G. <go...@gm...>]

Hello.

I would like to know if there's a way to suppress the
'Cross-reference-target ("entry") does not exist' error for entries without
locators.

First, consider the following index entry:

    \index{entry|see{non-existent entry}}

When xindy builds the index, it will complain about the dangling
cross-reference:

    ERROR: Cross-reference-target ("non-existent entry") does not exist!

We can instruct xindy not to validate the 'see' and 'seealso'
cross-references by adding the following to a custom .xdy module:

    (define-crossref-class "see" :unverified)
    (define-crossref-class "seealso" :unverified)

and passing the name of that module to xindy using the -M parameter.

I like having xindy validate my cross-references so I don't have dangling
pointers in my index. Unfortunately, it gives me false errors in the
following case:

    \index{entry!subentry}
    \index{another entry|see{entry}}

xindy complains:

    ERROR: Cross-reference-target ("entry") does not exist!

I assume this is because 'entry' has no locators (page numbers) itself and
only has subentries.

How can I get xindy to mark cross-references to such locatorless entries as
valid?

(If you'd like to earn fake Internet points for answering this question,
I've also posted it at <https://tex.stackexchange.com/q/424722/80>.)

Thanks for your help!

—Kevin Godby