Drop restrictions in widenReachCaptures

These should be no longer necessary with existentials.

Author: odersky

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

@@ -381,11 +381,13 @@ extension (tp: Type)
           t.dealias match
             case t1 @ CapturingType(p, cs) if cs.isUniversal && !isFlipped =>
               t1.derivedCapturingType(apply(p), ref.reach.singletonCaptureSet)
-            case t @ FunctionOrMethod(args, res @ Existential(_, _))
+            case t1 @ FunctionOrMethod(args, res @ Existential(_, _))
             if args.forall(_.isAlwaysPure) =>
               // Also map existentials in results to reach capabilities if all
               // preceding arguments are known to be always pure
-              apply(t.derivedFunctionOrMethod(args, Existential.toCap(res)))
+              apply(t1.derivedFunctionOrMethod(args, Existential.toCap(res)))
+            case Existential(_, _) =>
+              t
             case _ => t match
               case t @ CapturingType(p, cs) =>
                 t.derivedCapturingType(apply(p), cs) // don't map capture set variables
@@ -397,7 +399,7 @@ extension (tp: Type)
     ref match
       case ref: CaptureRef if ref.isTrackableRef =>
         val checker = new CheckContraCaps
-        checker.traverse(tp)
+        if !ccConfig.useExistentials then checker.traverse(tp)
         if checker.ok then
           val tp1 = narrowCaps(tp)
           if tp1 ne tp then capt.println(i"narrow $tp of $ref to $tp1")

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

@@ -1103,7 +1103,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             ccConfig.allowUniversalInBoxed
             || expected.hasAnnotation(defn.UncheckedCapturesAnnot)
             || actual.widen.hasAnnotation(defn.UncheckedCapturesAnnot)
-          if criticalSet.isUniversal && expected.isValueType && !allowUniversalInBoxed then
+          if criticalSet.isUnboxable && expected.isValueType && !allowUniversalInBoxed then
             // We can't box/unbox the universal capability. Leave `actual` as it is
             // so we get an error in checkConforms. Add the error message generated
             // from boxing as an addendum. This tends to give better error

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

@@ -12,9 +12,10 @@
    |                           Required: IO^ -> IO^{x*}
    |
    | longer explanation available when compiling with `-explain`
--- Error: tests/neg-custom-args/captures/widen-reach.scala:8:18 --------------------------------------------------------
-8 |trait Bar extends Foo[IO^]: // error
-  |                  ^
-  |                  IO^{ex$3} cannot be box-converted to box IO^
-  |                  since at least one of their capture sets contains the root capability `cap`
-9 |  val foo: IO^ -> IO^ = x => x
+-- [E164] Declaration Error: tests/neg-custom-args/captures/widen-reach.scala:9:6 --------------------------------------
+9 |  val foo: IO^ -> IO^ = x => x // error
+  |      ^
+  |      error overriding value foo in trait Foo of type IO^ -> box IO^;
+  |        value foo of type IO^ -> (ex$3: caps.Exists) -> IO^{ex$3} has incompatible type
+  |
+  | longer explanation available when compiling with `-explain`

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

@@ -5,8 +5,8 @@ trait IO
 trait Foo[+T]:
   val foo: IO^ -> T
 
-trait Bar extends Foo[IO^]: // error
-  val foo: IO^ -> IO^ = x => x
+trait Bar extends Foo[IO^]:
+  val foo: IO^ -> IO^ = x => x // error
 
 def test(x: Foo[IO^]): Unit =
   val y1: Foo[IO^{x*}] = x