Interpolator inheritance over-specificity required

[ivan-gomes]

When using an @InterpolatorFunction (and perhaps elsewhere) the referenced (Proto)Interpolator object must have as its first subtype ProtoInterpolator<T, R>. It results in an error,

Error transforming expression: Fatal Exception: The interpolator type InterpolatingAppender has no 2nd argument (i.e. the output-type)

if either:

1. ProtoInterpolator is the second subtype, e.g. object HtmlAppender : Any(), ProtoInterpolator<Any?, Unit>
2. ProtoInterpolator is inherited, e.g.

object HtmlAppender : InterpolatingAppender()

abstract class InterpolatingAppender : ProtoInterpolator<Any?, Unit>

A workaround is to explicitly re-specify a subtyping of ProtoInterpolator as the first subtype of the object, e.g.

object HtmlAppender : ProtoInterpolator<Any?, Unit>, InterpolatingAppender()

References:

• https://kotlinlang.slack.com/archives/C07CVTQ6WBG/p1727082314481859?thread_ts=1726792917.767779&cid=C07CVTQ6WBG
• https://github.com/TwingineerHQ/ktemplar/blob/develop/src/commonMain/kotlin/com/twingineer/ktemplar/Html.kt#L15