Make error reporting resilient to exception thrown while reporting (#20158)

Previously the added test failed with `1 error reported` but no actual
error message printed, because a stack overflow is thrown while
reporting the original error. This is then caught and handled to emit a
RecursionOverflow error, but that second error is non-sensical and
non-sensical errors are only printed if `hasErrors` returns false.

We fix this by deferring incrementing the error count (and therefore
having `hasErrors` return true) until after having displayed the error.
We also defer calling `markReported` otherwise the second error will
also be suppressed. A similar change is necessary in our testing
infrastructure to keep the error count is coherent.

Author: odersky

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

@@ -15,6 +15,7 @@ import java.io.{BufferedReader, PrintWriter}
 import scala.annotation.internal.sharable
 import scala.collection.mutable
 import core.Decorators.em
+import core.handleRecursive
 
 object Reporter {
   /** Convert a SimpleReporter into a real Reporter */
@@ -155,6 +156,12 @@ 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
+        try
+          withMode(Mode.Printing)(doReport(dia))
+        catch case ex: Throwable =>
+          // #20158: Don't increment the error count, otherwise we might suppress
+          // the RecursiveOverflow error and not print any error at all.
+          handleRecursive("error reporting", dia.message, ex)
         dia match {
           case w: Warning =>
             warnings = w :: warnings
@@ -168,7 +175,6 @@ abstract class Reporter extends interfaces.ReporterResult {
           // match error if d is something else
         }
         markReported(dia)
-        withMode(Mode.Printing)(doReport(dia))
   end issueUnconfigured
 
   def issueIfNotSuppressed(dia: Diagnostic)(using Context): Unit =

compiler/test/dotty/tools/dotc/reporting/TestReporter.scala

@@ -71,8 +71,8 @@ extends Reporter with UniqueMessagePositions with HideNonSensicalMessages with M
     }
 
     if dia.level >= WARNING then
-      _diagnosticBuf.append(dia)
       _consoleReporter.doReport(dia)
+      _diagnosticBuf.append(dia)
     printMessageAndPos(dia, extra)
   }
 }

tests/neg/mt-deskolemize.scala

@@ -0,0 +1,16 @@
+trait Expr:
+  type Value
+object Expr:
+  type Of[V] = Expr { type Value = V }
+  type ExtractValue[F <: Expr] = F match
+    case Expr.Of[v] => v
+import Expr.ExtractValue
+
+class SimpleLoop1 extends Expr:
+  type Value = ExtractValue[SimpleLoop2]
+
+class SimpleLoop2 extends Expr:
+  type Value = ExtractValue[SimpleLoop1]
+
+object Test1:
+  val x: ExtractValue[SimpleLoop1] = 1 // error