From 64dadd64594b13b77edf0d49c394e1b296fb07e4 Mon Sep 17 00:00:00 2001
From: bitloi <89318445+bitloi@users.noreply.github.com>
Date: Fri, 23 Jan 2026 11:41:16 -0500
Subject: [PATCH] [2.x] fix: Throw on addCompilerPlugin(foo % Test) (#8622)

Problem

When using `addCompilerPlugin((dependency) % Test)`, the compiler plugin was incorrectly added to BOTH `test:scalacOptions` AND `compile:scalacOptions`, instead of only `test:scalacOptions`.

Solution

Throw when the dependency is scoped, since we do not support this use case.
---
 main/src/main/scala/sbt/Defaults.scala | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index dcd16d260..67f2bd8fc 100644
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -4642,8 +4642,19 @@ trait BuildExtra extends BuildCommon with DefExtra {
     }
 
   /** Transforms `dependency` to be in the auto-compiler plugin configuration. */
-  def compilerPlugin(dependency: ModuleID): ModuleID =
-    dependency.withConfigurations(Some("plugin->default(compile)"))
+  def compilerPlugin(dependency: ModuleID): ModuleID = {
+    dependency.configurations match {
+      case Some(confs) if confs.toLowerCase != "compile" && confs.nonEmpty =>
+        sys.error(
+          s"""Configuration-scoped compiler plugins are not supported.
+             |Found: addCompilerPlugin(... % $confs)
+             |Use: addCompilerPlugin(...) without configuration scope.
+             |The plugin will be applied to all configurations.""".stripMargin
+        )
+      case _ =>
+        dependency.withConfigurations(Some("plugin->default(compile)"))
+    }
+  }
 
   /** Adds `dependency` to `libraryDependencies` in the auto-compiler plugin configuration. */
   def addCompilerPlugin(dependency: ModuleID): Setting[Seq[ModuleID]] =