Insert a space between symbol and `:' when printing SelectionProtos

Previously, we'd see something like `? { :+: Int }`, which is confusing.
Now we see instead `? { :+ : Int }`.

Author: odersky

src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -148,7 +148,8 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       case JavaArrayType(elemtp) =>
         return toText(elemtp) ~ "[]"
       case tp: SelectionProto =>
-        return "?{ " ~ toText(tp.name) ~ ": " ~ toText(tp.memberProto) ~ " }"
+        return "?{ " ~ toText(tp.name) ~ (" " provided !tp.name.decode.last.isLetterOrDigit) ~
+          ": " ~ toText(tp.memberProto) ~ " }"
       case tp: ViewProto =>
         return toText(tp.argType) ~ " ?=>? " ~ toText(tp.resultType)
       case tp @ FunProto(args, resultType, _) =>