Kotlin version mismatch possible when using bindings from server

[krzema12]

Context

Kotlin compiler's version used by the bindings server is regularly updated, and generally tracks the newest one. Also, the bindings server is the default, recommended way of consuming action bindings.

Problem

If someone uses a Kotlin version that is not compatible with the one that the bindings server used to compile the bindings, the compiler complains:

error: module was compiled with an incompatible version of Kotlin. The binary version of its metadata is 2.1.0, expected version is 1.8.0. (main.kotlin_module)
error: incompatible classes were found in dependencies. Remove them from the classpath or use '-Xskip-metadata-version-check' to suppress errors
.github/workflows/branches-and-prs.main.kts:24:59: error: class 'io.github.typesafegithub.workflows.actions.actions.Checkout' was compiled with an incompatible version of Kotlin. The actual metadata version is 2.1.0, but the compiler version 1.8.0 can read versions up to 1.9.0.
The class is loaded from /Users/piotr/.m2/repository/actions/checkout/v4/checkout-v4.jar!/io/github/typesafegithub/workflows/actions/actions/Checkout.class
import io.github.typesafegithub.workflows.actions.actions.Checkout
                                                          ^

Ideas

• use the newest Kotlin version on the server, but tell it to produce output with fixed metadata (possibly to support a few years in the past?)
  • not sure if Kotlin gives us such an option
• expose multiple artifacts/versions/server routes, to give the user the control of the version of Kotlin the bindings are compiled with
  • this can easily get unbearably complex, TBD

[LeoColman]

In Java we usually do

• Compile with Java 8
• Run on Java 250 no problem

Can't we use Kotlin 1.5 to compile things on the server and that would provide enough compatibility? Naturally we don't need to go that low, but backwards compatibility should be possible

---

Maybe Kotlin has something like "CompileVersion"/"SourceVersion" same thing as Java?

[krzema12]

Yes, that's the idea I'd like to try out first, perhaps I'll reorder the ideas in the issue description.

[krzema12]

Asked on Slack: https://kotlinlang.slack.com/archives/C7L3JB43G/p1735652698014899