Drop special handling of functions with pure arguments in Existential.toCap

If existentials are mapped to fresh, it matters where they are opened. Pure or not
arguments don't have anything to do with that.

Author: odersky

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

@@ -242,18 +242,10 @@ object Existential:
       case _ =>
         core
 
-  /** Map top-level existentials to `cap`. Do the same for existentials
-   *  in function results if all preceding arguments are known to be always pure.
-   */
+  /** Map top-level existentials to `cap`. */
   def toCap(tp: Type)(using Context): Type = tp.dealiasKeepAnnots match
     case Existential(boundVar, unpacked) =>
-      val transformed = unpacked.substParam(boundVar, defn.captureRoot.termRef)
-      transformed match
-        case FunctionOrMethod(args, res @ Existential(_, _))
-        if args.forall(_.isAlwaysPure) =>
-          transformed.derivedFunctionOrMethod(args, toCap(res))
-        case _ =>
-          transformed
+      unpacked.substParam(boundVar, defn.captureRoot.termRef)
     case tp1 @ CapturingType(parent, refs) =>
       tp1.derivedCapturingType(toCap(parent), refs)
     case tp1 @ AnnotatedType(parent, ann) =>