Add useJCenter setting, which is set to false. Fixes #2217

Adds a new setting `useJCenter`, which is set to `false` by default.
When set to `true`, JCenter will be placed as the first external
resolver to find library dependencies.

The implementation of `externalResolvers` is changed to incorporate the
setting by calling `Resolver.reorganizeAppResolvers`. These changes
were required because `externalResolvers` uses whatever that's in the
launchconfig, which the build user may not upgrade.

main/src/main/scala/sbt/Defaults.scala

@@ -1148,6 +1148,7 @@ object Classpaths {
     moduleConfigurations :== Nil,
     publishTo :== None,
     resolvers :== Nil,
+    useJCenter :== false,
     retrievePattern :== Resolver.defaultRetrievePattern,
     transitiveClassifiers :== Seq(SourceClassifier, DocClassifier),
     sourceArtifactTypes :== Artifact.DefaultSourceTypes,
@@ -1174,10 +1175,10 @@ object Classpaths {
     organizationHomepage <<= organizationHomepage or homepage,
     projectInfo <<= (name, description, homepage, startYear, licenses, organizationName, organizationHomepage, scmInfo, developers) apply ModuleInfo,
     overrideBuildResolvers <<= appConfiguration(isOverrideRepositories),
-    externalResolvers <<= (externalResolvers.task.?, resolvers, appResolvers) {
-      case (Some(delegated), Seq(), _) => delegated
-      case (_, rs, Some(ars))          => task { ars ++ rs } // TODO - Do we need to filter out duplicates?
-      case (_, rs, _)                  => task { Resolver.withDefaultResolvers(rs) }
+    externalResolvers <<= (externalResolvers.task.?, resolvers, appResolvers, useJCenter) {
+      case (Some(delegated), Seq(), _, _) => delegated
+      case (_, rs, Some(ars), uj)         => task { ars ++ Resolver.reorganizeAppResolvers(rs, uj, true) }
+      case (_, rs, _, uj)                 => task { Resolver.withDefaultResolvers(rs, uj) }
     },
     appResolvers <<= appConfiguration apply appRepositories,
     bootResolvers <<= appConfiguration map bootRepositories,

main/src/main/scala/sbt/Keys.scala

@@ -345,6 +345,7 @@ object Keys {
   val projectResolver = TaskKey[Resolver]("project-resolver", "Resolver that handles inter-project dependencies.", DTask)
   val fullResolvers = TaskKey[Seq[Resolver]]("full-resolvers", "Combines the project resolver, default resolvers, and user-defined resolvers.", CTask)
   val otherResolvers = SettingKey[Seq[Resolver]]("other-resolvers", "Resolvers not included in the main resolver chain, such as those in module configurations.", CSetting)
+  val useJCenter = SettingKey[Boolean]("use-jcenter", "Use JCenter as the default repository.", BSetting)
   val moduleConfigurations = SettingKey[Seq[ModuleConfiguration]]("module-configurations", "Defines module configurations, which override resolvers on a per-module basis.", BMinusSetting)
   val retrievePattern = SettingKey[String]("retrieve-pattern", "Pattern used to retrieve managed dependencies to the current build.", DSetting)
   val retrieveConfiguration = SettingKey[Option[RetrieveConfiguration]]("retrieve-configuration", "Configures retrieving dependencies to the current build.", DSetting)

notes/0.13.10/make-JCenter-opt-in.markdown

@@ -0,0 +1,7 @@
+
+  [@eed3si9n]: https://github.com/eed3si9n
+  [2217]: https://github.com/sbt/sbt/issues/2217
+
+### Fixes with compatibility implications
+
+- sbt 0.13.10 adds a new setting `useJCenter`, which is set to `false` by default. When set to `true`, JCenter will be placed as the first external resolver to find library dependencies. [#2217][2217] by [@eed3si9n][@eed3si9n]

sbt/src/sbt-test/dependency-management/default-resolvers/build.sbt

@@ -0,0 +1,16 @@
+lazy val check = taskKey[Unit]("")
+lazy val check2 = taskKey[Unit]("")
+
+lazy val root = (project in file(".")).
+  settings(
+    check := {
+      val fr = fullResolvers.value
+      assert(!(fr exists { _.name == "jcenter" }))
+      assert(fr exists { _.name == "public" })
+    },
+    check2 := {
+      val fr = fullResolvers.value
+      assert(fr exists { _.name == "jcenter" })
+      assert(fr exists { _.name == "public" })
+    }
+  )

sbt/src/sbt-test/dependency-management/default-resolvers/test

@@ -0,0 +1,5 @@
+> check
+
+> set useJCenter := true
+
+> check2