SI-6440 Address regressions around MissingRequirementError

 Go back to using globalError to report when a stub's info is referenced,
 and only throw the MissingRequirementError when compilation really
 must abort due to having a StubTermSymbol in a place where a
 StubClassSymbol would have been a better choice.

 This situation arises when an entire package is missing from the
 classpath, as was the case in the reported bug.

 Adds `StoreReporterDirectTest`, which buffers messages issued
 during compilation for more structured interrogation. Use this
 in two test for manifests -- these tests were using a crude means
 of grepping compiler console output to focus on the relevant output,
 but this approach was insufficient with the new multi-line error
 message emitted as part of this change.

 Also used that base test class to add two new tests: one for
 the reported error (package missing), and another for a simpler
 error (class missing). The latter test shows how stub symbols
 allow code to compile if it doesn't the subset of signatures
 in some type that refer to a missing class.

 Gave the INFO/WARNING/ERROR members of Reporter sensible
 toString implementations; they inherit from Enumeration#Value
 in an unusual manner (why?) that means the built in toString of
 Enumeration printed `Severity@0`.

6 files changed, 47 insertions, 15 deletions

diff --git a/src/compiler/scala/tools/nsc/reporters/Reporter.scala b/src/compiler/scala/tools/nsc/reporters/Reporter.scala
index c5321dd728..8871ae6555 100644
--- a/src/compiler/scala/tools/nsc/reporters/Reporter.scala
+++ b/src/compiler/scala/tools/nsc/reporters/Reporter.scala
@@ -20,9 +20,15 @@ abstract class Reporter {
   class Severity(val id: Int) extends severity.Value {
     var count: Int = 0
   }
-  val INFO    = new Severity(0)
-  val WARNING = new Severity(1)
-  val ERROR   = new Severity(2)
+  val INFO    = new Severity(0) {
+    override def toString: String = "INFO"
+  }
+  val WARNING = new Severity(1) {
+    override def toString: String = "WARNING"
+  }
+  val ERROR   = new Severity(2) {
+    override def toString: String = "ERROR"
+  }
 
   /** Whether very long lines can be truncated.  This exists so important
    *  debugging information (like printing the classpath) is not rendered
diff --git a/src/partest/scala/tools/partest/DirectTest.scala b/src/partest/scala/tools/partest/DirectTest.scala
index 554e7848c7..8c18809ad6 100644
--- a/src/partest/scala/tools/partest/DirectTest.scala
+++ b/src/partest/scala/tools/partest/DirectTest.scala
@@ -8,6 +8,7 @@ package scala.tools.partest
 import scala.tools.nsc._
 import io.Directory
 import util.{BatchSourceFile, CommandLineParser}
+import reporters.{Reporter, ConsoleReporter}
 
 /** A class for testing code which is embedded as a string.
  *  It allows for more complete control over settings, compiler
@@ -38,9 +39,12 @@ abstract class DirectTest extends App {
   // new compiler
   def newCompiler(args: String*): Global = {
     val settings = newSettings((CommandLineParser tokenize ("-d \"" + testOutput.path + "\" " + extraSettings)) ++ args.toList)
-    if (settings.Yrangepos.value) new Global(settings) with interactive.RangePositions
-    else new Global(settings)
+    if (settings.Yrangepos.value) new Global(settings, reporter(settings)) with interactive.RangePositions
+    else new Global(settings, reporter(settings))
   }
+
+  def reporter(settings: Settings): Reporter = new ConsoleReporter(settings)
+
   def newSources(sourceCodes: String*) = sourceCodes.toList.zipWithIndex map {
     case (src, idx) => new BatchSourceFile("newSource" + (idx + 1), src)
   }
diff --git a/src/partest/scala/tools/partest/StoreReporterDirectTest.scala b/src/partest/scala/tools/partest/StoreReporterDirectTest.scala
new file mode 100644
index 0000000000..7f3604c86c
--- /dev/null
+++ b/src/partest/scala/tools/partest/StoreReporterDirectTest.scala
@@ -0,0 +1,15 @@
+package scala.tools.partest
+
+import scala.tools.nsc.Settings
+import scala.tools.nsc.reporters.StoreReporter
+import scala.collection.mutable
+
+trait StoreReporterDirectTest extends DirectTest {
+  lazy val storeReporter: StoreReporter = new scala.tools.nsc.reporters.StoreReporter()
+
+  /** Discards all but the first message issued at a given position. */
+  def filteredInfos: Seq[storeReporter.Info] = storeReporter.infos.groupBy(_.pos).map(_._2.head).toList
+
+  /** Hook into [[scala.tools.partest.DirectTest]] to install the custom reporter */
+  override def reporter(settings: Settings) = storeReporter
+}
diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index 67b2d05224..06bf54c1f8 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -3103,12 +3103,18 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     )
   }
   trait StubSymbol extends Symbol {
-    override final def failIfStub() = fail(())
     protected def missingMessage: String
+
+    /** Fail the stub by throwing a [[scala.reflect.internal.MissingRequirementError]]. */
+    override final def failIfStub() = {MissingRequirementError.signal(missingMessage)} //
+
+    /** Fail the stub by reporting an error to the reporter, setting the IS_ERROR flag
+      * on this symbol, and returning the dummy value `alt`.
+      */
     private def fail[T](alt: T): T = {
       // Avoid issuing lots of redundant errors
       if (!hasFlag(IS_ERROR)) {
-        MissingRequirementError.signal(missingMessage)
+        globalError(missingMessage)
         if (settings.debug.value)
           (new Throwable).printStackTrace
 
diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index 57212bab55..d5347705a6 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -1385,10 +1385,11 @@ trait Types extends api.Types { self: SymbolTable =>
   /** A class for this-types of the form <sym>.this.type
    */
   abstract case class ThisType(sym: Symbol) extends SingletonType with ThisTypeApi {
-    // SI-6640 allow StubSymbols to reveal what's missing from the classpath
-    //         before we trip the assertion.
-    sym.failIfStub()
-    assert(sym.isClass, sym)
+    if (!sym.isClass) {
+      // SI-6640 allow StubSymbols to reveal what's missing from the classpath before we trip the assertion.
+      sym.failIfStub()
+      assert(false, sym)
+    }
 
     //assert(sym.isClass && !sym.isModuleClass || sym.isRoot, sym)
     override def isTrivial: Boolean = sym.isPackageClass
diff --git a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
index 5464a68205..f3a5053a91 100644
--- a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
+++ b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
@@ -234,10 +234,10 @@ abstract class UnPickler {
               adjust(mirrorThatLoaded(owner).missingHook(owner, name)) orElse {
                 // (5) Create a stub symbol to defer hard failure a little longer.
                 val missingMessage =
-                  s"""|A signature in $filename refers to ${name.longString} in ${owner.fullLocationString}
-                      |which is not available. It may be completely missing from the current classpath,
-                      |or the version on the classpath might be incompatible with the version used when
-                      |compiling $filename.""".stripMargin
+                  s"""|bad symbolic reference. A signature in $filename refers to ${name.longString}
+                      |in ${owner.kindString} ${owner.fullName} which is not available.
+                      |It may be completely missing from the current classpath, or the version on
+                      |the classpath might be incompatible with the version used when compiling $filename.""".stripMargin
                 owner.newStubSymbol(name, missingMessage)
               }
             }