diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index a5e114b81..fa9b6f509 100644
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -180,6 +180,7 @@ object Defaults extends BuildCommon {
       apiMappings := Map.empty,
       autoScalaLibrary :== true,
       managedScalaInstance :== true,
+      allowUnsafeScalaLibUpgrade :== false,
       classpathEntryDefinesClass := { (file: File) =>
         sys.error("use classpathEntryDefinesClassVF instead")
       },
@@ -1178,6 +1179,7 @@ object Defaults extends BuildCommon {
   def scalaInstanceFromUpdate: Initialize[Task[ScalaInstance]] = Def.task {
     val sv = scalaVersion.value
     val fullReport = update.value
+    val s = streams.value
 
     // For Scala 3, update scala-library.jar in `scala-tool` and `scala-doc-tool` in case a newer version
     // is present in the `compile` configuration. This is needed once forwards binary compatibility is dropped
@@ -1204,24 +1206,38 @@ object Defaults extends BuildCommon {
     )
 
     if (Classpaths.isScala213(sv)) {
-      for {
-        compileReport <- fullReport.configuration(Configurations.Compile)
-        libName <- ScalaArtifacts.Artifacts
-      } {
-        for (lib <- compileReport.modules.find(_.module.name == libName)) {
-          val libVer = lib.module.revision
-          val n = name.value
-          if (VersionNumber(sv).matchesSemVer(SemanticSelector(s"<$libVer")))
-            sys.error(
-              s"""expected `$n/scalaVersion` to be "$libVer" or later,
-                 |but found "$sv"; upgrade scalaVersion to fix the build.
-                 |
-                 |to support backwards-only binary compatibility (SIP-51),
-                 |the Scala 2.13 compiler cannot be older than $libName on the
-                 |dependency classpath.
-                 |see `$n/evicted` to know why $libName $libVer is getting pulled in.
-                 |""".stripMargin
-            )
+      val scalaDeps = for {
+        compileReport <- fullReport.configuration(Configurations.Compile).iterator
+        libName <- ScalaArtifacts.Artifacts.iterator
+        lib <- compileReport.modules.find(_.module.name == libName)
+      } yield lib
+      for (lib <- scalaDeps.take(1)) {
+        val libVer = lib.module.revision
+        val libName = lib.module.name
+        val n = name.value
+        if (VersionNumber(sv).matchesSemVer(SemanticSelector(s"<$libVer"))) {
+          val err = !allowUnsafeScalaLibUpgrade.value
+          val fix =
+            if (err)
+              """Upgrade the `scalaVersion` to fix the build. If upgrading the Scala compiler version is
+                |not possible (for example due to a regression in the compiler or a missing dependency),
+                |this error can be demoted by setting `allowUnsafeScalaLibUpgrade := true`.""".stripMargin
+            else
+              s"""Note that the dependency classpath and the runtime classpath of your project
+                 |contain the newer $libName $libVer, even if the scalaVersion is $sv.
+                 |Compilation (macro expansion) or using the Scala REPL in sbt may fail with a LinkageError.""".stripMargin
+
+          val msg =
+            s"""Expected `$n/scalaVersion` to be $libVer or later, but found $sv.
+               |To support backwards-only binary compatibility (SIP-51), the Scala 2.13 compiler
+               |should not be older than $libName on the dependency classpath.
+               |
+               |$fix
+               |
+               |See `$n/evicted` to know why $libName $libVer is getting pulled in.
+               |""".stripMargin
+          if (err) sys.error(msg)
+          else s.log.warn(msg)
         }
       }
     }
diff --git a/main/src/main/scala/sbt/Keys.scala b/main/src/main/scala/sbt/Keys.scala
index 9349773d7..94b1f7def 100644
--- a/main/src/main/scala/sbt/Keys.scala
+++ b/main/src/main/scala/sbt/Keys.scala
@@ -571,6 +571,7 @@ object Keys {
   val conflictManager = settingKey[ConflictManager]("Selects the conflict manager to use for dependency management.").withRank(CSetting)
   val autoScalaLibrary = settingKey[Boolean]("Adds a dependency on scala-library if true.").withRank(ASetting)
   val managedScalaInstance = settingKey[Boolean]("Automatically obtains Scala tools as managed dependencies if true.").withRank(BSetting)
+  val allowUnsafeScalaLibUpgrade = settingKey[Boolean]("Allow the Scala library on the compilation classpath to be newer than the scalaVersion (see Scala SIP-51).").withRank(CSetting)
   val sbtResolver = settingKey[Resolver]("Provides a resolver for obtaining sbt as a dependency.").withRank(BMinusSetting)
   val sbtResolvers = settingKey[Seq[Resolver]]("The external resolvers for sbt and plugin dependencies.").withRank(BMinusSetting)
   val sbtDependency = settingKey[ModuleID]("Provides a definition for declaring the current version of sbt.").withRank(BMinusSetting)
diff --git a/main/src/main/scala/sbt/internal/LintUnused.scala b/main/src/main/scala/sbt/internal/LintUnused.scala
index c2d2db006..8df675c15 100644
--- a/main/src/main/scala/sbt/internal/LintUnused.scala
+++ b/main/src/main/scala/sbt/internal/LintUnused.scala
@@ -34,6 +34,7 @@ object LintUnused {
       commands,
       crossScalaVersions,
       crossSbtVersions,
+      allowUnsafeScalaLibUpgrade,
       initialize,
       lintUnusedKeysOnLoad,
       onLoad,
diff --git a/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/a/A.scala b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/a/A.scala
new file mode 100644
index 000000000..c66551e1b
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/a/A.scala
@@ -0,0 +1,27 @@
+import scala.language.reflectiveCalls
+
+
+package scala.collection.immutable {
+  object Exp {
+    // Access RedBlackTree.validate added in Scala 2.13.13
+    def v = RedBlackTree.validate(null)(null)
+  }
+}
+
+
+object A extends App {
+  println(scala.util.Properties.versionString)
+}
+
+object AMacro {
+  import scala.language.experimental.macros
+  import scala.reflect.macros.blackbox.Context
+
+  def m(x: Int): Int = macro impl
+
+  def impl(c: Context)(x: c.Expr[Int]): c.Expr[Int] = {
+    import c.universe._
+    println(scala.collection.immutable.Exp.v)
+    c.Expr(q"2 + $x")
+  }
+}
diff --git a/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/b/B.scala b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/b/B.scala
new file mode 100644
index 000000000..f75b7905e
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/b/B.scala
@@ -0,0 +1,7 @@
+import java.nio.file.{Paths, Files}
+import java.nio.charset.StandardCharsets
+
+object B extends App {
+  println(AMacro.m(33)) // fails
+  Files.write(Paths.get(s"s${scala.util.Properties.versionNumberString}.txt"), "nix".getBytes)
+}
diff --git a/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/build.sbt b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/build.sbt
new file mode 100644
index 000000000..c757f2648
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/build.sbt
@@ -0,0 +1,39 @@
+import sbt.librarymanagement.InclExclRule
+
+lazy val a = project.settings(
+  scalaVersion := "2.13.13",
+  libraryDependencies += "org.scala-lang" % "scala-reflect" % scalaVersion.value,
+  TaskKey[Unit]("checkLibs") := checkLibs("2.13.13", (Compile/dependencyClasspath).value, ".*scala-(library|reflect).*"),
+)
+
+lazy val b = project.dependsOn(a).settings(
+  allowUnsafeScalaLibUpgrade := true,
+  scalaVersion := "2.13.12",
+
+  // dependencies are upgraded to 2.13.13
+  TaskKey[Unit]("checkLibs") := checkLibs("2.13.13", (Compile/dependencyClasspath).value, ".*scala-(library|reflect).*"),
+
+  // check the compiler uses the 2.13.12 library on its runtime classpath
+  TaskKey[Unit]("checkScala") := {
+    val i = scalaInstance.value
+    i.libraryJars.filter(_.toString.contains("scala-library")).toList match {
+      case List(l) => assert(l.toString.contains("2.13.12"), i.toString)
+    }
+    assert(i.compilerJars.filter(_.toString.contains("scala-library")).isEmpty, i.toString)
+    assert(i.otherJars.filter(_.toString.contains("scala-library")).isEmpty, i.toString)
+  },
+)
+
+lazy val c = project.dependsOn(a).settings(
+  allowUnsafeScalaLibUpgrade := true,
+  scalaVersion := "2.13.12",
+  TaskKey[Unit]("checkLibs") := checkLibs("2.13.13", (Compile/dependencyClasspath).value, ".*scala-(library|reflect).*"),
+)
+
+def checkLibs(v: String, cp: Classpath, filter: String): Unit = {
+  for (p <- cp)
+    if (p.toString.matches(filter)) {
+      println(s"$p -- $v")
+      assert(p.toString.contains(v), p)
+    }
+}
diff --git a/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/c/C.scala b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/c/C.scala
new file mode 100644
index 000000000..de0f7c084
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/c/C.scala
@@ -0,0 +1,7 @@
+import java.nio.file.{Paths, Files}
+import java.nio.charset.StandardCharsets
+
+object C extends App {
+  assert(scala.collection.immutable.Exp.v == null)
+  Files.write(Paths.get(s"s${scala.util.Properties.versionNumberString}.txt"), "nix".getBytes)
+}
diff --git a/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/test b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/test
new file mode 100644
index 000000000..f347b3534
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-warn/test
@@ -0,0 +1,11 @@
+> a/checkLibs
+> b/checkLibs
+> b/checkScala
+> c/checkLibs
+
+# macro expansion fails
+-> b/compile
+
+> c/run
+$ exists s2.13.13.txt
+$ delete s2.13.13.txt