Also print boxes for variable capture sets with no elems defined

Author: odersky

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -208,7 +208,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
         else if ctx.settings.YccDebug.value then
           changePrec(GlobalPrec)(box ~ refs.show ~ " " ~ toText(parent))
         else if !refs.isConst && refs.elems.isEmpty then
-          changePrec(GlobalPrec)("?" ~ " " ~ toText(parent))
+          changePrec(GlobalPrec)(box ~ "?" ~ " " ~ toText(parent))
         else if Config.printCaptureSetsAsPrefix then
           changePrec(GlobalPrec)(box ~ toText(refs) ~ " " ~ toText(parent))
         else

tests/neg-custom-args/captures/boxmap.check

@@ -1,7 +1,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/boxmap.scala:12:2 ----------------------------------------
 12 |  () => b[Box[B]]((x: A) => box(f(x)))  // error
    |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |  Found:    {f} () -> ? Box[? B]
+   |  Found:    {f} () -> ? Box[box ? B]
    |  Required: () -> Box[B]
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/lazylist.check

@@ -37,6 +37,6 @@
 17 |  def tail = xs() // error: cannot have an inferred type
    |  ^^^^^^^^^^^^^^^
    |  Non-local method tail cannot have an inferred result type
-   |  {LazyCons.this.xs} lazylists.LazyList[? T]
+   |  {LazyCons.this.xs} lazylists.LazyList[box ? T]
    |  with non-empty capture set {LazyCons.this.xs}.
    |  The type needs to be declared explicitly.

tests/neg-custom-args/captures/lazylists2.check

@@ -8,7 +8,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists2.scala:18:4 ------------------------------------
 18 |    final class Mapped extends LazyList[B]:  // error
    |    ^
-   |    Found:    {f, xs} LazyList[? B]
+   |    Found:    {f, xs} LazyList[box ? B]
    |    Required: {f} LazyList[B]
 19 |      this: ({xs, f} Mapped) =>
 20 |      def isEmpty = false
@@ -20,7 +20,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists2.scala:27:4 ------------------------------------
 27 |    final class Mapped extends LazyList[B]:  // error
    |    ^
-   |    Found:    {f, xs} LazyList[? B]
+   |    Found:    {f, xs} LazyList[box ? B]
    |    Required: {xs} LazyList[B]
 28 |      this: ({xs, f} Mapped) =>
 29 |      def isEmpty = false