Fixes now that explicit nulls are on

Author: dwijnand

compiler/src/dotty/tools/dotc/core/Decorators.scala

@@ -249,7 +249,7 @@ object Decorators {
   extension [T](x: T)
     def showing[U](
         op: WrappedResult[U] ?=> String,
-        printer: config.Printers.Printer = config.Printers.default)(using c: Conversion[T, U] = null): T = {
+        printer: config.Printers.Printer = config.Printers.default)(using c: Conversion[T, U] | Null = null): T = {
       // either the use of `$result` was driven by the expected type of `Shown`
       // which led to the summoning of `Conversion[T, Shown]` (which we'll invoke)
       // or no such conversion was found so we'll consume the result as it is instead
@@ -268,7 +268,7 @@ object Decorators {
               if !ctx.mode.is(Mode.PrintShowExceptions) && !ctx.settings.YshowPrintErrors.value =>
             val msg = ex match { case te: TypeError => te.toMessage case _ => ex.getMessage }
             s"[cannot display due to $msg, raw string = $x]"
-      case _ => String.valueOf(x)
+      case _ => String.valueOf(x).nn
 
   extension [T](x: T)
     def assertingErrorsReported(using Context): T = {

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

@@ -50,6 +50,9 @@ object Formatting {
       given [A: Show, B: Show]: Show[(A, B)] with
         def show(x: (A, B)) = (Show[A].show(x._1), Show[B].show(x._2))
 
+      given [X: Show]: Show[X | Null] with
+        def show(x: X | Null) = if x == null then "null" else Show[X].show(x.nn)
+
       given Show[FlagSet] with
         def show(x: FlagSet) = x.flagsString