Don't have Diagnostic inherit from Exception

Creating diagnostics should be cheap, whereas reportiong them can be expensive.
The reason is that often diagnsotics are created nd then later discarded in normal
backtracking during Typer.

But the way it was set up, every diagnostic computed a stack trace, which is quite expensive.

Author: odersky

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

@@ -89,7 +89,7 @@ class Diagnostic(
   val msg: Message,
   val pos: SourcePosition,
   val level: Int
-) extends Exception with interfaces.Diagnostic:
+) extends interfaces.Diagnostic:
   private var verbose: Boolean = false
   def isVerbose: Boolean = verbose
   def setVerbose(): this.type =
@@ -102,5 +102,4 @@ class Diagnostic(
     msg.message.replaceAll("\u001B\\[[;\\d]*m", "")
 
   override def toString: String = s"$getClass at $pos: $message"
-  override def getMessage(): String = message
 end Diagnostic

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

@@ -6,12 +6,16 @@ import core.Contexts._
 import Diagnostic.Error
 
 /**
- * This class implements a Reporter that throws all errors and sends warnings and other
- * info to the underlying reporter.
+ * This class implements a Reporter that throws all errors as UnhandledError exceptions
+ * and sends warnings and other info to the underlying reporter.
  */
 class ThrowingReporter(reportInfo: Reporter) extends Reporter {
   def doReport(dia: Diagnostic)(using Context): Unit = dia match {
-    case _: Error => throw dia
+    case dia: Error => throw UnhandledError(dia)
     case _ => reportInfo.doReport(dia)
   }
 }
+
+class UnhandledError(val diagnostic: Error) extends Exception:
+  override def getMessage = diagnostic.message
+

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -2401,7 +2401,7 @@ trait Applications extends Compatibility {
       else
         None
     catch
-      case NonFatal(_) => None
+      case ex: UnhandledError => None
 
   def isApplicableExtensionMethod(methodRef: TermRef, receiverType: Type)(using Context): Boolean =
     methodRef.symbol.is(ExtensionMethod) && !receiverType.isBottomType &&