Add an Append.Sequence instance for List, fixes #2107

main/settings/src/main/scala/sbt/Append.scala

@@ -23,6 +23,14 @@ object Append {
     def appendValues(a: Seq[T], b: Seq[V]): Seq[T] = a ++ (b map { x => (x: T) })
     def appendValue(a: Seq[T], b: V): Seq[T] = a :+ (b: T)
   }
+  implicit def appendList[T, V <: T]: Sequence[List[T], List[V], V] = new Sequence[List[T], List[V], V] {
+    def appendValues(a: List[T], b: List[V]): List[T] = a ::: b
+    def appendValue(a: List[T], b: V): List[T] = a :+ b
+  }
+  implicit def appendListImplicit[T, V <% T]: Sequence[List[T], List[V], V] = new Sequence[List[T], List[V], V] {
+    def appendValues(a: List[T], b: List[V]): List[T] = a ::: (b map { x => (x: T) })
+    def appendValue(a: List[T], b: V): List[T] = a :+ (b: T)
+  }
   implicit def appendString: Value[String, String] = new Value[String, String] {
     def appendValue(a: String, b: String) = a + b
   }

main/settings/src/test/scala/UsageTest.scala

@@ -38,6 +38,8 @@ object Assign {
   val dummy3 = settingKey[complete.Parser[(String, Int)]]("dummy3")
   val tsk: complete.Parser[Task[String]] = ???
   val itsk: Initialize[InputTask[Int]] = ???
+  val seqSetting = settingKey[Seq[String]]("seqSetting")
+  val listSetting = settingKey[List[String]]("listSetting")
 
   /*	def azy = sk.value
 
@@ -100,4 +102,14 @@ object Assign {
   def forallIn[T](key: Initialize[T]): Initialize[Seq[T]] = Def.setting {
     key.value :: Nil
   }
+
+  // Test that Append.Sequence instances for Seq/List work and don't mess up with each other
+  seqSetting := Seq("test1")
+  seqSetting ++= Seq("test2")
+  seqSetting ++= List("test3")
+  seqSetting += "test4"
+
+  listSetting := List("test1")
+  listSetting ++= List("test2")
+  listSetting += "test4"
 }

notes/0.13.10/add-append.sequence-list.markdown

@@ -0,0 +1,11 @@
+[@pdalpra]: http://github.com/pdalpra
+[2107]: https://github.com/sbt/sbt/issues/2107
+[2114]: https://github.com/sbt/sbt/pull/2114
+
+### Fixes with compatibility implications
+
+### Improvements
+
+### Bug fixes
+
+- Add an Append.Sequence instance for List, allow +=/++= on `developers` SettingKey [#2107][2107]/[#2114][2114] by [@pdalpra][@pdalpra]