Call hasErrors first but then bump before doReport

We can switch to a more straightforward hasErrors check in isHidden, but
then we need to bump the errorCount before calling doReport, as doReport
will then, necessarily, force the message.

For reference, the way I test this manually is by:
  1. In ignoredConvertibleImplicits, changing back to `viewExists(imp, fail.expectedType)`
  2. In adapt1, removing the `dummyTreeOfType.unapply(tree).isEmpty` guard
  3. Compiling tests/neg/i18650.scala

Also, while I'm here, instruct on the presence of
-Yno-enrich-error-messages, like we do for -Yno-decode-stacktraces.

compiler/src/dotty/tools/dotc/report.scala

@@ -155,6 +155,7 @@ object report:
        |  An unhandled exception was thrown in the compiler.
        |  Please file a crash report here:
        |  https://github.com/lampepfl/dotty/issues/new/choose
+       |  For non-enriched exceptions, compile with -Yno-enrich-error-messages.
        |
        |$info1
        |""".stripMargin

compiler/src/dotty/tools/dotc/reporting/HideNonSensicalMessages.scala

@@ -13,19 +13,8 @@ trait HideNonSensicalMessages extends Reporter {
    */
   override def isHidden(dia: Diagnostic)(using Context): Boolean =
     super.isHidden(dia) || {
-        (if !errorsReported && dia.isInstanceOf[Diagnostic.Error] then
-          // Bump up errorCount so hasErrors is true while forcing the message.
-          // We use errorsReported as a predicate for broken code.
-          // So now any forcing won't cause, for instance,
-          // assertion errors and thus compiler crashes.
-          // Some messages, once forced, run more code
-          // to generate useful hints for the user.
-          try
-            _errorCount += 1
-            dia.msg.isNonSensical
-          finally _errorCount -= 1 // decrease rather than reset the value so we only ever decrease by 1
-        else dia.msg.isNonSensical) &&
-        hasErrors && // if there are no errors yet, report even if diagnostic is non-sensical
-        !ctx.settings.YshowSuppressedErrors.value
+        hasErrors  // if there are no errors yet, report even if diagnostic is non-sensical
+        && dia.msg.isNonSensical // defer forcing the message by calling hasErrors first
+        && !ctx.settings.YshowSuppressedErrors.value
     }
 }

compiler/src/dotty/tools/dotc/reporting/Reporter.scala

@@ -92,8 +92,8 @@ abstract class Reporter extends interfaces.ReporterResult {
 
   private def isIncompleteChecking = incompleteHandler ne defaultIncompleteHandler
 
-  protected var _errorCount = 0
-  protected var _warningCount = 0
+  private var _errorCount = 0
+  private var _warningCount = 0
 
   /** The number of errors reported by this reporter (ignoring outer reporters) */
   def errorCount: Int = _errorCount
@@ -155,8 +155,6 @@ abstract class Reporter extends interfaces.ReporterResult {
       addUnreported(key, 1)
     case _                                                  =>
       if !isHidden(dia) then // avoid isHidden test for summarized warnings so that message is not forced
-        markReported(dia)
-        withMode(Mode.Printing)(doReport(dia))
         dia match {
           case w: Warning =>
             warnings = w :: warnings
@@ -169,6 +167,8 @@ abstract class Reporter extends interfaces.ReporterResult {
           case _: Info    => // nothing to do here
           // match error if d is something else
         }
+        markReported(dia)
+        withMode(Mode.Printing)(doReport(dia))
   end issueUnconfigured
 
   def issueIfNotSuppressed(dia: Diagnostic)(using Context): Unit =