Make empty capture sets of Mutable types have Reader mutability

Was Ignored before.

Author: odersky

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

@@ -624,8 +624,7 @@ object CaptureSet:
     private var isComplete = true
 
     def associateWithMutable()(using Context): Unit =
-      if !elems.isEmpty then
-        isComplete = false // delay computation of Mutability status
+      isComplete = false // delay computation of Mutability status
 
     override def mutability(using Context): Mutability =
       if !isComplete then

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

@@ -1484,6 +1484,9 @@ class CheckCaptures extends Recheck, SymTransformer:
           else
             trace.force(i"rechecking $tree with pt = $pt", recheckr, show = true):
               super.recheck(tree, pt)
+        catch case ex: AssertionError =>
+          println(i"error while rechecking $tree against $pt")
+          throw ex
         finally curEnv = saved
       if tree.isTerm && !pt.isBoxedCapturing && pt != LhsProto then
         markFree(res.boxedCaptureSet, tree)
@@ -1781,7 +1784,7 @@ class CheckCaptures extends Recheck, SymTransformer:
           val cs = actual.captureSet
           if covariant then cs ++ leaked
           else
-            if !leaked.subCaptures(cs) then
+            if !leaked.isAlwaysEmpty && !leaked.subCaptures(cs) then
               report.error(
                 em"""$expected cannot be box-converted to ${actual.capturing(leaked)}
                     |since the additional capture set $leaked resulting from box conversion is not allowed in $actual""", tree.srcPos)