A more flexible scheme for handling the universal capability (#18699)

- Start with `sealed` type parameters, extended so that class parameters
can also be `sealed`.
- Tighten the rules so that type parameter arguments for `sealed` type
parameters must themselves be `sealed`, except if the argument is from
some outer scope.
- Introduce a coercion that maps a universal capture set to a local root
that is enclosed by the owners of all free variables of the coerced
expression.


Supersedes #18566

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -4,7 +4,6 @@ package dotc
 import core._
 import Contexts._
 import typer.{TyperPhase, RefChecks}
-import cc.CheckCaptures
 import parsing.Parser
 import Phases.Phase
 import transform._
@@ -85,8 +84,8 @@ class Compiler {
          new PatternMatcher) ::      // Compile pattern matches
     List(new TestRecheck.Pre) ::     // Test only: run rechecker, enabled under -Yrecheck-test
     List(new TestRecheck) ::         // Test only: run rechecker, enabled under -Yrecheck-test
-    List(new CheckCaptures.Pre) ::   // Preparations for check captures phase, enabled under captureChecking
-    List(new CheckCaptures) ::       // Check captures, enabled under captureChecking
+    List(new cc.Setup) ::            // Preparations for check captures phase, enabled under captureChecking
+    List(new cc.CheckCaptures) ::    // Check captures, enabled under captureChecking
     List(new ElimOpaque,             // Turn opaque into normal aliases
          new sjs.ExplicitJSClasses,  // Make all JS classes explicit (Scala.js only)
          new ExplicitOuter,          // Add accessors to outer classes from nested ones.

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

@@ -437,7 +437,7 @@ object desugar {
   private def toDefParam(tparam: TypeDef, keepAnnotations: Boolean): TypeDef = {
     var mods = tparam.rawMods
     if (!keepAnnotations) mods = mods.withAnnotations(Nil)
-    tparam.withMods(mods & EmptyFlags | Param)
+    tparam.withMods(mods & (EmptyFlags | Sealed) | Param)
   }
   private def toDefParam(vparam: ValDef, keepAnnotations: Boolean, keepDefault: Boolean): ValDef = {
     var mods = vparam.rawMods

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

@@ -810,7 +810,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     }
   }
 
-  /** An extractor for def of a closure contained the block of the closure,
+  /** An extractor for the method of a closure contained the block of the closure,
    *  possibly with type ascriptions.
    */
   object possiblyTypedClosureDef:

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

@@ -1286,6 +1286,21 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
       !(sym.is(Method) && sym.info.isInstanceOf[MethodOrPoly]) // if is a method it is parameterless
   }
 
+  /** A tree traverser that generates the the same import contexts as original typer for statements.
+   *  TODO: Should we align TreeMapWithPreciseStatContexts and also keep track of exprOwners?
+   */
+  abstract class TreeTraverserWithPreciseImportContexts extends TreeTraverser:
+    override def apply(x: Unit, trees: List[Tree])(using Context): Unit =
+      def recur(trees: List[Tree]): Unit = trees match
+        case (imp: Import) :: rest =>
+          traverse(rest)(using ctx.importContext(imp, imp.symbol))
+        case tree :: rest =>
+          traverse(tree)
+          traverse(rest)
+        case Nil =>
+      recur(trees)
+  end TreeTraverserWithPreciseImportContexts
+
   extension (xs: List[tpd.Tree])
     def tpes: List[Type] = xs match {
       case x :: xs1 => x.tpe :: xs1.tpes

compiler/src/dotty/tools/dotc/cc/CaptureAnnotation.scala

@@ -24,8 +24,17 @@ case class CaptureAnnotation(refs: CaptureSet, boxed: Boolean)(cls: Symbol) exte
   import CaptureAnnotation.*
   import tpd.*
 
-  /** A cache for boxed version of a capturing type with this annotation */
-  val boxedType = BoxedTypeCache()
+  /** A cache for the version of this annotation which differs in its boxed status. */
+  var boxDual: CaptureAnnotation | Null = null
+
+  /** A boxed annotation which is either the same annotation or its boxDual */
+  def boxedAnnot(using Context): CaptureAnnotation =
+    if boxed then this
+    else if boxDual != null then boxDual.nn
+    else
+      val dual = CaptureAnnotation(refs, boxed = true)(cls)
+      dual.boxDual = this
+      dual
 
   /** Reconstitute annotation tree from capture set */
   override def tree(using Context) =