Fix well-formed test for capabilities (#23393)

 - Disallow cap*
 - Do allow f*.rd

Author: noti0na1

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

@@ -98,24 +98,23 @@ object CheckCaptures:
    *  This check is performed at Typer.
    */
   def checkWellformed(parent: Tree, ann: Tree)(using Context): Unit =
-    def check(elem: Type, pos: SrcPos): Unit = elem match
-      case ref: Capability =>
+    def check(elem: Type): Unit = elem match
+      case ref: TypeRef =>
+        val refSym = ref.symbol
+        if refSym.isType && !refSym.info.derivesFrom(defn.Caps_CapSet) then
+          report.error(em"$elem is not a legal element of a capture set", ann.srcPos)
+      case ref: CoreCapability =>
         if !ref.isTrackableRef && !ref.isCapRef then
-          report.error(em"$elem cannot be tracked since it is not a parameter or local value", pos)
+          report.error(em"$elem cannot be tracked since it is not a parameter or local value", ann.srcPos)
+      case ReachCapability(ref) =>
+        check(ref)
+        if ref.isCapRef then
+          report.error(em"Cannot form a reach capability from `cap`", ann.srcPos)
+      case ReadOnlyCapability(ref) =>
+        check(ref)
       case tpe =>
-        report.error(em"$elem: $tpe is not a legal element of a capture set", pos)
-    for elem <- ann.retainedSet.retainedElementsRaw do
-      elem match
-        case ref: TypeRef =>
-          val refSym = ref.symbol
-          if refSym.isType && !refSym.info.derivesFrom(defn.Caps_CapSet) then
-            report.error(em"$elem is not a legal element of a capture set", ann.srcPos)
-        case ReachCapability(ref) =>
-          check(ref, ann.srcPos)
-        case ReadOnlyCapability(ref) =>
-          check(ref, ann.srcPos)
-        case _ =>
-          check(elem, ann.srcPos)
+        report.error(em"$elem: $tpe is not a legal element of a capture set", ann.srcPos)
+    ann.retainedSet.retainedElementsRaw.foreach(check)
 
   /** Under the sealed policy, report an error if some part of `tp` contains the
    *  root capability in its capture set or if it refers to a type parameter that

tests/neg-custom-args/captures/caps-reach.check

@@ -0,0 +1,20 @@
+-- [E040] Syntax Error: tests/neg-custom-args/captures/caps-reach.scala:9:46 -------------------------------------------
+9 |  val consumers4 = ListBuffer.empty[() ->{f.rd*} Unit] // error
+  |                                              ^
+  |                                              '}' expected, but identifier found
+-- [E040] Syntax Error: tests/neg-custom-args/captures/caps-reach.scala:10:46 ------------------------------------------
+10 |  val consumers5 = ListBuffer.empty[() ->{f.rd.rd} Unit] // error
+   |                                              ^
+   |                                              '}' expected, but '.' found
+-- [E040] Syntax Error: tests/neg-custom-args/captures/caps-reach.scala:11:46 ------------------------------------------
+11 |  val consumers6 = ListBuffer.empty[() ->{f * *} Unit] // error
+   |                                              ^
+   |                                              '}' expected, but identifier found
+-- Error: tests/neg-custom-args/captures/caps-reach.scala:6:42 ---------------------------------------------------------
+6 |  val consumers1 = ListBuffer.empty[() ->{caps.cap*} Unit]  // error
+  |                                          ^^^^^^^^^
+  |                                          Cannot form a reach capability from `cap`
+-- Error: tests/neg-custom-args/captures/caps-reach.scala:7:42 ---------------------------------------------------------
+7 |  val consumers2 = ListBuffer.empty[() ->{caps.cap*.rd} Unit] // error
+  |                                          ^^^^^^^^^^^^
+  |                                          Cannot form a reach capability from `cap`

tests/neg-custom-args/captures/caps-reach.scala

@@ -0,0 +1,11 @@
+import language.experimental.captureChecking
+import scala.collection.mutable.ListBuffer
+
+class MyContainer:
+  val f: Object^ = ???
+  val consumers1 = ListBuffer.empty[() ->{caps.cap*} Unit]  // error
+  val consumers2 = ListBuffer.empty[() ->{caps.cap*.rd} Unit] // error
+  val consumers3 = ListBuffer.empty[() ->{f*.rd} Unit] // ok
+  val consumers4 = ListBuffer.empty[() ->{f.rd*} Unit] // error
+  val consumers5 = ListBuffer.empty[() ->{f.rd.rd} Unit] // error
+  val consumers6 = ListBuffer.empty[() ->{f * *} Unit] // error