Use Java release option

build.sbt

@@ -473,9 +473,6 @@ lazy val core = crossProject(JSPlatform, JVMPlatform)
       } else Seq()
     }
   )
-  .jvmSettings(
-    javacOptions ++= Seq("-source", "1.8", "-target", "1.8")
-  )
   .jsSettings(
     libraryDependencies += "org.scala-js" %%% "scala-js-macrotask-executor" % MacrotaskExecutorVersion)
 
@@ -604,4 +601,7 @@ lazy val benchmarks = project
       "-Dcats.effect.tracing.exceptions.enhanced=false"))
   .enablePlugins(NoPublishPlugin, JmhPlugin)
 
-lazy val docs = project.in(file("site-docs")).dependsOn(core.jvm).enablePlugins(MdocPlugin)
+lazy val docs = project
+  .in(file("site-docs"))
+  .dependsOn(core.jvm)
+  .enablePlugins(MdocPlugin)

project/Java8Target.scala

@@ -0,0 +1,41 @@
+import sbt._, Keys._
+import sbtspiewak.SpiewakPlugin, SpiewakPlugin.autoImport._
+
+object Java8Target extends AutoPlugin {
+
+  override def requires = SpiewakPlugin
+  override def trigger = allRequirements
+
+  override def projectSettings = Seq(
+    scalacOptions ++= {
+      val version = System.getProperty("java.version")
+
+      // The release flag controls what JVM platform APIs are called. We only want JDK 8 APIs
+      // in order to maintain compability with JDK 8.
+      val releaseFlag =
+        if (version.startsWith("1.8"))
+          Seq()
+        else
+          Seq("-release", "8")
+
+      // The target flag is not implied by `-release` on Scala 2. We need to set it explicitly.
+      // The target flag controls the JVM bytecode version that is output by scalac.
+      val targetFlag =
+        if (isDotty.value || version.startsWith("1.8"))
+          Seq()
+        else if (scalaVersion.value.startsWith("2.12"))
+          Seq("-target:jvm-1.8")
+        else
+          Seq("-target:8")
+
+      releaseFlag ++ targetFlag
+    },
+
+    javacOptions ++= {
+      val version = System.getProperty("java.version")
+      if (version.startsWith("1.8"))
+        Seq()
+      else
+        Seq("--release", "8")
+    })
+}