Don't widen arguments for tracked class parameters

Author: odersky

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

@@ -40,6 +40,9 @@ object CheckCaptures:
   val name: String = "cc"
   val description: String = "capture checking"
 
+  /** An attachment to prevent widening of arguments to tracked parameters */
+  val NoWiden: Property.Key[Unit] = Property.Key()
+
   enum EnvKind derives CanEqual:
     case Regular        // normal case
     case NestedInOwner  // environment is a temporary one nested in the owner's environment,
@@ -829,6 +832,7 @@ class CheckCaptures extends Recheck, SymTransformer:
       //   - the selection is either a trackable capture reference or a pure type, or
       //   - if the selection is of a parameterless method capturing a ResultCap
       if noWiden(selType, pt)
+          || tree.hasAttachment(NoWiden)
           || qualType.isBoxedCapturing
           || selType.isBoxedCapturing
           || selWiden.isBoxedCapturing
@@ -885,6 +889,11 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  TODO: Maybe not charge deep capture sets for consume?
      */
     protected override def recheckArg(arg: Tree, formal: Type, pref: ParamRef, app: Apply)(using Context): Type =
+      val meth = app.fun.symbol
+      if meth.isPrimaryConstructor
+          && meth.owner.asClass.refiningGetterNamed(pref.paramName).is(Tracked)
+      then
+        arg.putAttachment(NoWiden, ())
       val instantiatedFormal = globalCapToLocal(formal, Origin.Formal(pref, app))
       val argType = recheck(arg, instantiatedFormal)
         .showing(i"recheck arg $arg vs $instantiatedFormal = $result", capt)
@@ -1980,7 +1989,7 @@ class CheckCaptures extends Recheck, SymTransformer:
      *   - do box adaptation
      */
     def adapt(actual: Type, expected: Type, tree: Tree)(using Context): Type =
-      if noWiden(actual, expected) then
+      if noWiden(actual, expected) || tree.removeAttachment(NoWiden).isDefined then
         expected match
           case expected @ CapturingType(_, _) if expected.isBoxed =>
             // actual is a singleton type and expected is of the form box x.type^cs.

tests/neg-custom-args/captures/i25251.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg-custom-args/captures/i25251.scala:5:9 --------------------------------------------------------------
+5 |    this.g() // error
+  |    ^^^^^^
+  |    Cannot call update method g of A.this
+  |    since the access is in method f, which is not an update method.
+-- Error: tests/neg-custom-args/captures/i25251.scala:6:8 --------------------------------------------------------------
+6 |    ref.g()  // error
+  |    ^^^^^
+  |    Cannot call update method g of ref
+  |    since the access is in method f, which is not an update method.

tests/neg-custom-args/captures/i25251.scala

@@ -2,7 +2,7 @@ import caps.*
 trait A extends Stateful:
   def f() = /* read-only f */
     val ref: this.type = this
-    this.g()
-    ref.g()
+    this.g() // error
+    ref.g()  // error
 
   update def g() = ???

tests/pos-custom-args/captures/i25251.scala

@@ -0,0 +1,13 @@
+import language.experimental.captureChecking
+import language.experimental.modularity
+import caps.*
+
+trait Collection[+T] extends Stateful:
+  self: Collection[T]^ =>
+    type Index
+
+    // slice should have its Index type member a subtype of the Collection's Index
+    def getSlice(): (Collection[T] { type Index <: self.Index })^{this.rd} = new Slice(this)
+
+class Slice[+T](tracked val coll: Collection[T]) extends Collection[T]:
+  type Index = coll.Index