Cross-compile to Scala 2.11

Author: guilgaly

.ci_scripts/validate.sh

@@ -15,4 +15,4 @@ false
 
 echo "[INFO] Running tests" >> /dev/stdout
 
-sbt ++$TRAVIS_SCALA_VERSION testOnly
+sbt ++$TRAVIS_SCALA_VERSION test

.travis.yml

@@ -1,8 +1,10 @@
 sudo: false
 language: scala
+scala:
+  - 2.12.7
+  - 2.11.12
 jdk:
   - oraclejdk8
-scala: 2.12.7
 cache:
   directories:
   - $HOME/.ivy2

build.sbt

@@ -2,13 +2,11 @@ name := "query-monad-code"
 
 version := "1.0-SNAPSHOT"
 
-scalaVersion in ThisBuild := "2.12.7"
+ThisBuild / scalaVersion := "2.12.7"
+ThisBuild / crossScalaVersions := Seq("2.11.12", "2.12.7")
 
-// Common values
-val commonSettings = Seq(
-  organization := "com.zengularity",
-  crossPaths := false,
-  scalacOptions ++= Seq(
+def scalacOptionsVersion(scalaVersion: String) = {
+  val defaultOptions = Seq(
     "-deprecation", // Emit warning and location for usages of deprecated APIs.
     "-encoding",
     "utf-8", // Specify character encoding used by source files.
@@ -21,15 +19,32 @@ val commonSettings = Seq(
     "-Yno-adapted-args", // Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.
     "-Ypartial-unification", // Enable partial unification in type constructor inference
     "-Ywarn-dead-code", // Warn when dead code is identified.
-    "-Ywarn-extra-implicit", // Warn when more than one implicit parameter section is defined.
     "-Ywarn-inaccessible", // Warn about inaccessible types in method signatures.
     "-Ywarn-infer-any", // Warn when a type argument is inferred to be `Any`.
     "-Ywarn-nullary-override", // Warn when non-nullary `def f()' overrides nullary `def f'.
     "-Ywarn-nullary-unit", // Warn when nullary methods return Unit.
     "-Ywarn-numeric-widen", // Warn when numerics are widened.
     "-Ywarn-unused", // Warn if unused.
     "-Ywarn-value-discard" // Warn when non-Unit expression results are unused.
-  ),
+  )
+  val v211Options = Seq(
+    "-Xsource:2.12" // See https://github.com/scala/scala/releases/tag/v2.11.11
+  )
+  val v212Options = Seq(
+    "-Ywarn-extra-implicit", // Warn when more than one implicit parameter section is defined.
+  )
+  
+  CrossVersion.partialVersion(scalaVersion) match {
+    case Some((2L, 11L)) => defaultOptions ++ v211Options
+    case _ => defaultOptions ++ v212Options
+  }
+}
+
+// Common values
+def commonSettings = Seq(
+  organization := "com.zengularity",
+  crossPaths := false,
+  scalacOptions ++= scalacOptionsVersion(scalaVersion.value),
   scalacOptions in (Compile, console) ~= (_.filterNot(
     Set(
       "-Ywarn-unused:imports",