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.

3 files changed, 15 insertions, 3 deletions

diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index 33d99b35d8..67b2d05224 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -1344,6 +1344,9 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
       }
     }
 
+    /** Raises a `MissingRequirementError` if this symbol is a `StubSymbol` */
+    def failIfStub() {}
+
     /** Initialize the symbol */
     final def initialize: this.type = {
       if (!isInitialized) info
@@ -3100,6 +3103,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     )
   }
   trait StubSymbol extends Symbol {
+    override final def failIfStub() = fail(())
     protected def missingMessage: String
     private def fail[T](alt: T): T = {
       // Avoid issuing lots of redundant errors
diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index 86d03d7450..57212bab55 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -1385,7 +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 {
-    assert(sym.isClass, {sym.info; sym}) // call .info to allow StubSymbols to reveal what's missing from the classpath
+    // SI-6640 allow StubSymbols to reveal what's missing from the classpath
+    //         before we trip the assertion.
+    sym.failIfStub()
+    assert(sym.isClass, sym)
+
     //assert(sym.isClass && !sym.isModuleClass || sym.isRoot, sym)
     override def isTrivial: Boolean = sym.isPackageClass
     override def isNotNull = true
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)
               }
             }