Refine the message and triggering of MissingRequirementError.

 - To force a failure of the stub, call a new method `failIfStub`
   rather than `info`.
 - Offer a broader range of potential root causes in the
   error message.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
index 15465f919a..5464a68205 100644
--- a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
+++ b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
@@ -20,7 +20,7 @@ import scala.annotation.switch
 /** @author Martin Odersky
  *  @version 1.0
  */
-abstract class UnPickler /*extends scala.reflect.generic.UnPickler*/ {
+abstract class UnPickler {
   val global: SymbolTable
   import global._
 
@@ -233,7 +233,11 @@ abstract class UnPickler /*extends scala.reflect.generic.UnPickler*/ {
               // (4) Call the mirror's "missing" hook.
               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 missing from the classpath."
+                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
                 owner.newStubSymbol(name, missingMessage)
               }
             }