From c8b2b03f8a12ecada4740967b9d0a4abbcc03b30 Mon Sep 17 00:00:00 2001
From: eugene yokota <eed3si9n@gmail.com>
Date: Sat, 14 Feb 2026 01:04:40 -0800
Subject: [PATCH] [1.x] Print warning about scala-reflect not found (#8736)

**Problem**
scala-reflect not found issue is confusing.

**Solution**
Print out a specialized warning message.
---
 .../sbt/internal/LibraryManagement.scala      | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/main/src/main/scala/sbt/internal/LibraryManagement.scala b/main/src/main/scala/sbt/internal/LibraryManagement.scala
index c50609ed8..a069eebbb 100644
--- a/main/src/main/scala/sbt/internal/LibraryManagement.scala
+++ b/main/src/main/scala/sbt/internal/LibraryManagement.scala
@@ -146,6 +146,11 @@ private[sbt] object LibraryManagement {
             .apply(updateInputs)
           if (isCached) markAsCached(report) else report
         } catch {
+          case r: ResolveException
+              if r.failed.exists(isMissingScalaLib) &&
+                module.scalaModuleInfo.exists(_.scalaBinaryVersion == "2.13") =>
+            informSandwich()
+            throw r
           case t @ (_: NullPointerException | _: OutOfMemoryError) =>
             val resolvedAgain = resolve
             val culprit = t.getClass.getSimpleName
@@ -160,6 +165,27 @@ private[sbt] object LibraryManagement {
       Tracked.inputChanged(cacheStoreFactory.make("inputs"))(doCachedResolve)
     }
 
+    def informSandwich(): Unit = {
+      log.warn("[sbt-8728] Smorrebrod - the end of Scala 2.13-3.x sandwich")
+      log.warn("")
+      log.warn("Scala 3.8+ cannot be used in a Scala 2.13 subproject.")
+      log.warn(
+        "Dependency resolution failed because scala-reflect or -compiler 3.x does not exist."
+      )
+      log.warn(
+        "This happens when a Scala 2.13 subproject depends on Scala 3.8+ directly or transitively."
+      )
+      log.warn("To fix this, either")
+      log.warn("  - Keep Scala 3 subproject or transitive dependency to 3.7 or below, or")
+      log.warn("  - Migrate the Scala 2.13 subproject to Scala 3.x")
+      log.warn("See https://github.com/sbt/sbt/discussions/8728")
+    }
+
+    def isMissingScalaLib(m: ModuleID): Boolean =
+      m.organization == "org.scala-lang" &&
+        (m.name == "scala-compiler" || m.name == "scala-reflect") &&
+        (m.revision.startsWith("3."))
+
     // Get the handler to use and feed it in the inputs
     // This is lm-engine specific input hashed into Long
     val extraInputHash = module.extraInputHash