[BUG][kotlin] Undocumented reserved words: entries, keys, size, values

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

The following words act as reserved words for Kotlin, but this fact is not documented anywhere: "entries", "keys", "size", "values". Hence, it's also not clear why?

It took me some debugging to find out that it was done in PR #10071 with the following note:

    // model classes cannot use the same property names defined in HashMap
    // ref: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-hash-map/

openapi-generator version

5.4.0

OpenAPI declaration file content or url

This is trivial, just use any model with one of the mentioned reserved words as properties (or see steps to reproduce).

Generation Details

Use Kotlin library (either server or client - both are affected).

Steps to reproduce

Can even easily be reproduced in KotlinClientCodegenModelTest by adding, e.g., property "size" to the sample schema:

    .addProperties("size", new IntegerSchema())

Related issues/PRs

Initially introduced in PR #10071

Suggest a fix

Expected behavior: if this is somehow truly necessary for all models (is it?), it should at least be documented for all Kotlin generators (server and client).

Start with KotlinClientCodegenModelTest and its sample schema containing a property such as "size"; review how the Kotlin client and server generators treat the four reserved words. Done means the reserved words and the reason for their restriction are documented for both Kotlin generators.