New Syntax for Capture Variables and Explicit Capture Polymorphism

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -234,6 +234,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
 
     case class Tracked()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Tracked)
 
+    case class CaptureParam()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.CaptureParam)
+
     /** Used under pureFunctions to mark impure function types `A => B` in `FunctionWithMods` */
     case class Impure()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Impure)
   }
@@ -528,12 +530,13 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     TypeApply(Select(scalaDot(nme.caps), nme.capsOf), tp :: Nil)
 
   // Capture set variable `[C^]` becomes: `[C >: CapSet <: CapSet^{cap}]`
-  def makeCapsBound()(using Context): TypeBoundsTree =
-    TypeBoundsTree(
-      Select(scalaDot(nme.caps), tpnme.CapSet),
-      makeRetaining(
-        Select(scalaDot(nme.caps), tpnme.CapSet),
-        Nil, tpnme.retainsCap))
+  def makeCapsBound(refsL: List[Tree] = Nil, refsU: List[Tree] = Nil)(using Context): TypeBoundsTree =
+    val lower = refsL match
+      case Nil => Select(scalaDot(nme.caps), tpnme.CapSet)
+      case refsL => makeRetaining(Select(scalaDot(nme.caps), tpnme.CapSet), refsL, tpnme.retains)
+    val upper =
+      makeRetaining(Select(scalaDot(nme.caps), tpnme.CapSet), refsU, if refsU.isEmpty then tpnme.retainsCap else tpnme.retains)
+    TypeBoundsTree(lower, upper)
 
   def makeConstructor(tparams: List[TypeDef], vparamss: List[List[ValDef]], rhs: Tree = EmptyTree)(using Context): DefDef =
     DefDef(nme.CONSTRUCTOR, joinParams(tparams, vparamss), TypeTree(), rhs)

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -380,6 +380,9 @@ object Flags {
   /** Tracked modifier for class parameter / a class with some tracked parameters */
   val (Tracked @ _, _, Dependent @ _) = newFlags(46, "tracked")
 
+  /** Cap modifier for capture-set parameters and capture-set members */
+  val (_, _, CaptureParam @ _) = newFlags(47, "cap")
+
   // ------------ Flags following this one are not pickled ----------------------------------
 
   /** Symbol is not a member of its owner */

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -91,6 +91,14 @@ object Mode {
    */
   val ImplicitExploration: Mode = newMode(12, "ImplicitExploration")
 
+  /** We are currently inside a capture set.
+   *  A term name could be a capture variable, so we need to
+   *  check that it is valid to use as type name.
+   *  Since this mode is only used during annotation typing,
+   *  we can reuse the value of `ImplicitExploration` to save bits.
+   */
+  val InCaptureSet: Mode = ImplicitExploration
+
   /** We are currently unpickling Scala2 info */
   val Scala2Unpickling: Mode = newMode(13, "Scala2Unpickling")
 

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -443,6 +443,7 @@ object StdNames {
     val bytes: N                = "bytes"
     val canEqual_ : N           = "canEqual"
     val canEqualAny : N         = "canEqualAny"
+    val cap: N                  = "cap"
     val caps: N                 = "caps"
     val capsOf: N               = "capsOf"
     val captureChecking: N      = "captureChecking"