From 1bf6f0d2a0c55a57ac8d3f344ea438a06fa65a6b Mon Sep 17 00:00:00 2001
From: Eugene Yokota <eed3si9n@gmail.com>
Date: Sun, 15 Jul 2018 20:47:59 -0400
Subject: [PATCH] Make ++ fail when it doesn't affect any subprojects

Fixes #4267
---
 main/src/main/scala/sbt/Cross.scala                    | 6 ++++++
 sbt/src/sbt-test/actions/cross-strict-aggregation/test | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/main/src/main/scala/sbt/Cross.scala b/main/src/main/scala/sbt/Cross.scala
index 78571d78b..650e213f7 100644
--- a/main/src/main/scala/sbt/Cross.scala
+++ b/main/src/main/scala/sbt/Cross.scala
@@ -310,6 +310,12 @@ object Cross {
           case (_, scalaVersions) =>
             scalaVersions.exists(v => CrossVersion.binaryScalaVersion(v) == binaryVersion)
         }
+        if (included.isEmpty) {
+          sys.error(
+            s"""Switch failed: no subprojects list "$version" (or compatible version) in crossScalaVersions setting.
+               |If you want to force it regardless, call ++ $version!""".stripMargin
+          )
+        }
         logSwitchInfo(included, excluded)
         included
       }
diff --git a/sbt/src/sbt-test/actions/cross-strict-aggregation/test b/sbt/src/sbt-test/actions/cross-strict-aggregation/test
index fb3a4d9cb..2e9408232 100644
--- a/sbt/src/sbt-test/actions/cross-strict-aggregation/test
+++ b/sbt/src/sbt-test/actions/cross-strict-aggregation/test
@@ -1,3 +1,5 @@
+-> ++2.12.0-magic
+
 > ++2.11.11 compile
 
 $ exists core/target/scala-2.11