Fix unsafeAssumePure handling

Test against a fresh root var instead of against universal

Author: odersky

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

@@ -14,7 +14,7 @@ type CaptureRoot = TermRef | CaptureRoot.Var
 
 object CaptureRoot:
 
-  case class Var(owner: Symbol, source: Symbol)(using @constructorOnly ictx: Context) extends CaptureRef, Showable:
+  case class Var(owner: Symbol, source: Symbol = NoSymbol)(using @constructorOnly ictx: Context) extends CaptureRef, Showable:
 
     var upperBound: Symbol = owner
     var lowerBound: Symbol = NoSymbol

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

@@ -466,7 +466,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
       if meth == defn.Caps_unsafeAssumePure then
         val arg :: Nil = tree.args: @unchecked
-        val argType0 = recheck(arg, pt.capturing(CaptureSet.universal))
+        val argType0 = recheck(arg, pt.capturing(CaptureSet(CaptureRoot.Var(ctx.owner))))
         val argType =
           if argType0.captureSet.isAlwaysEmpty then argType0
           else argType0.widen.stripCapturing