Keep separate chain of outer reporters for StoreReporters

Used in rewritten `errorsReported` method. This fixes #866. The problem
before was that `TyperState#trywithFallBack` temporarily updates
the reporter, so crawling up the context chain to check for reporters
with errors missed some reported errors, which triggered the assertion
in `Tree#withType`.

Author: odersky

src/dotty/tools/dotc/Driver.scala

@@ -12,7 +12,7 @@ abstract class Driver extends DotClass {
 
   protected def newCompiler(): Compiler
 
-  protected def emptyReporter: Reporter = new StoreReporter
+  protected def emptyReporter: Reporter = new StoreReporter(null)
 
   protected def doCompile(compiler: Compiler, fileNames: List[String])(implicit ctx: Context): Reporter =
     if (fileNames.nonEmpty)

src/dotty/tools/dotc/ast/Trees.scala

@@ -146,7 +146,7 @@ object Trees {
      *  type. (Overridden by empty trees)
      */
     def withType(tpe: Type)(implicit ctx: Context): ThisTree[Type] = {
-      if (tpe == ErrorType) assert(ctx.errorsReported)
+      if (tpe == ErrorType) assert(ctx.reporter.errorsReported)
       withTypeUnchecked(tpe)
     }
 

src/dotty/tools/dotc/core/TyperState.scala

@@ -98,7 +98,7 @@ extends TyperState(r) {
 
 
   override def fresh(isCommittable: Boolean): TyperState =
-    new MutableTyperState(this, new StoreReporter, isCommittable)
+    new MutableTyperState(this, new StoreReporter(reporter), isCommittable)
 
   override def withReporter(reporter: Reporter) =
     new MutableTyperState(this, reporter, isCommittable)
@@ -169,7 +169,7 @@ extends TyperState(r) {
    *  found a better solution.
    */
   override def tryWithFallback[T](op: => T)(fallback: => T)(implicit ctx: Context): T = {
-    val storeReporter = new StoreReporter
+    val storeReporter = new StoreReporter(myReporter)
     val savedReporter = myReporter
     myReporter = storeReporter
     val savedConstraint = myConstraint

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

@@ -173,8 +173,6 @@ trait Reporting { this: Context =>
         throw ex
     }
   }
-
-  def errorsReported: Boolean = outersIterator exists (_.reporter.hasErrors)
 }
 
 /**
@@ -213,6 +211,11 @@ abstract class Reporter {
   def hasErrors = errorCount > 0
   def hasWarnings = warningCount > 0
 
+  /** Have errors been reported by this reporter, or in the
+   *  case where this is a StoreReporter, by an outer reporter?
+   */
+  def errorsReported = hasErrors
+
   val unreportedWarnings = new mutable.HashMap[String, Int] {
     override def default(key: String) = 0
   }

src/dotty/tools/dotc/reporting/StoreReporter.scala

@@ -10,7 +10,7 @@ import config.Printers._
 /**
  * This class implements a Reporter that stores all messages
  */
-class StoreReporter extends Reporter {
+class StoreReporter(outer: Reporter) extends Reporter {
 
   private var infos: mutable.ListBuffer[Diagnostic] = null
 
@@ -33,4 +33,6 @@ class StoreReporter extends Reporter {
       infos foreach ctx.reporter.report
       infos = null
     }
+
+  override def errorsReported = hasErrors || outer.errorsReported
 }