Can we perhaps go the extra mile and remove the [__local1__], too? If it's easy to do, figure out the prefix that is a function vs the tail of the ref... If it's cumbersome, forget about it, the new message is so much better than the old message already. This is just icing on the cake (I like icing on the cake).

Here you go some icing! Added a check to match the last part of the reference to the regex __local[0-9]+__ and drop it. Wish there was more context identifying what part is the function 🤔

but this does get us:

1 error occurred: test.rego:8: rego_type_error: function data.play.f used as reference, not called
        data.play.f[__local1__]
        ^^^^^^^^^^^
        have: (any) => any

There is also this logic that wraps the __local1__ in brackets that could be repurposed: https://github.com/open-policy-agent/opa/blob/main/v1/ast/term_appenders.go#L198-L223

Although it is vague enough that I am not sure what all the scenarios would be when something is wrapped in brackets. So I prefer the approach of handling this specific scenario until we find an example it doesn't work.