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I suspect a Build Tools API incremental compilation bug, see <https://github.com/com-lihaoyi/mill/pull/7152|https://github.com/com-lihaoyi/mill/pull/7152> for a repro. It could be in Mill internals but AFAIK the wrapper is quite lean

This is on 2.3.21 when a module you depend on adds a member to a sealed interface. Your `when` expressions should be recompiled to check exhaustiveness, but this doesn't seem to be happening

Please create an issue at youtrack (<http://kotl.in/issue|kotl.in/issue>).

<@UKPEELJCW> here <https://youtrack.jetbrains.com/issue/KT-86580/Build-Tools-API-incremental-compilation-doesnt-invalidate-when-sealed-type-changes|https://youtrack.jetbrains.com/issue/KT-86580/Build-Tools-API-incremental-compilation-doesnt-invalidate-when-sealed-type-changes>

Another Build Tools API issue <https://youtrack.jetbrains.com/issue/KT-86585/Build-Tools-API-incremental-compilation-fails-when-typealias-changes-to-a-value-class|https://youtrack.jetbrains.com/issue/KT-86585/Build-Tools-API-incremental-compilation-fails-when-typealias-changes-to-a-value-class> (didn't check whether it's BTA-specific though)

These MREs were done by AI but the bug is real, I just let Copilot reproduce problems we ran into in CI in the background. The original repo is proprietary

Seems very buggy from our brief (less than a week) experience with Build Tools API though