Add version-specific source directories in the library

And replace Tuple.scala with a Scala 2 implementation and a Scala 3
implementation. The Scala 2 implementaion of Tuple.scala is not strictly
necessary but could be useful if non-version-specific library sources
end up referencing scala.Tuple.

library/src/scala/Tuple.scala → library/src-scala2/scala/Tuple.scala

@@ -5,10 +5,10 @@ sealed trait Tuple extends Any
 
 @showAsInfix
 sealed class *:[+H, +T <: Tuple] extends Tuple {
-  @`rewrite` def head: H = ???
-  @`rewrite` def tail: T = ???
+  def head: H = ???
+  def tail: T = ???
 }
 
 object *: {
-  @`rewrite` def unapply[H, T <: Tuple](x: H *: T) = Some((x.head, x.tail))
+  def unapply[H, T <: Tuple](x: H *: T) = Some((x.head, x.tail))
 }

library/src-scala3/scala/Tuple.scala

@@ -0,0 +1,26 @@
+package scala
+import annotation.showAsInfix
+
+object typelevel {
+  erased def erasedValue[T]: T = ???
+}
+
+import typelevel._
+
+sealed trait Tuple extends Any
+
+@showAsInfix
+sealed class *:[+H, +T <: Tuple] extends Tuple {
+  rewrite def head: H = ???
+  rewrite def tail: T = ???
+
+  rewrite private def _size(xs: Tuple): Int = //rewrite
+    xs match {
+      case _: Unit => 0
+      case _: *:[_, xs1] => _size(erasedValue[xs1]) + 1
+    }
+}
+
+object *: {
+  rewrite def unapply[H, T <: Tuple](x: H *: T) = Some((x.head, x.tail))
+}

project/Build.scala

@@ -740,7 +740,17 @@ object Build {
 
   // Settings shared between dotty-library and dotty-library-bootstrapped
   lazy val dottyLibrarySettings = Seq(
-      libraryDependencies += "org.scala-lang" % "scala-library" % scalacVersion
+    libraryDependencies += "org.scala-lang" % "scala-library" % scalacVersion,
+    // Add version-specific source directories:
+    // - files in src-scala3 will only be compiled by dotty
+    // - files in src-scala2 will only be compiled by scalac
+    unmanagedSourceDirectories in Compile += {
+      val baseDir = baseDirectory.value
+      if (isDotty.value)
+        baseDir / "src-scala3"
+      else
+        baseDir / "src-scala2"
+    }
   )
 
   lazy val `dotty-library` = project.in(file("library")).asDottyLibrary(NonBootstrapped)