don't try to create tags w/o scala-reflect.jar

Since recently type tags have relocated to scala-reflect.jar,
meaning that they are no longer always on library classpath.

In the compiler we do have code that generates type tags, and this code
is bound to fail if scala-reflect.jar isn't there.

I though this wouldn't be a problem, because type tag materialization
is only going to be triggered by users explicitly requesting a type tag.

That's generally true, but I overlooked a corner case. Since we provide
manifest <-> type tag compatibility, manifest lookup can sometimes trigger
tag lookup, which might result in tag synthesis, which blows up like this:
http://groups.google.com/group/scala-internals/browse_thread/thread/166ce4b71b7c46bb

This commit also ensures that type tag generation/interop doesnt sneak into the
code of the libraries that don't have scala-reflect.jar on their classpath.
For details refer to the discussion at scala-internals:
http://groups.google.com/group/scala-internals/browse_thread/thread/72f6ce3010f4d8

1 files changed, 9 insertions, 6 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Tags.scala b/src/compiler/scala/tools/nsc/typechecker/Tags.scala
index 0dbeafadbe..d82fbd7c77 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Tags.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Tags.scala
@@ -59,11 +59,14 @@ trait Tags {
      *           EmptyTree if `concrete` is true and the result contains unresolved (i.e. not spliced) type parameters and abstract type members.
      *           EmptyTree if `allowMaterialization` is false, and there is no array tag in scope.
      */
-    def resolveTypeTag(pos: Position, pre: Type, tp: Type, concrete: Boolean, allowMaterialization: Boolean = true): Tree = {
-      val tagSym = if (concrete) TypeTagClass else WeakTypeTagClass
-      val tagTp =  if (pre == NoType) TypeRef(ApiUniverseClass.toTypeConstructor, tagSym, List(tp)) else singleType(pre, pre member tagSym.name)
-      val taggedTp = appliedType(tagTp, List(tp))
-      resolveTag(pos, taggedTp, allowMaterialization)
-    }
+    def resolveTypeTag(pos: Position, pre: Type, tp: Type, concrete: Boolean, allowMaterialization: Boolean = true): Tree =
+      // if someone requests a type tag, but scala-reflect.jar isn't on the library classpath, then bail
+      if (pre == NoType && ApiUniverseClass == NoSymbol) EmptyTree
+      else {
+        val tagSym = if (concrete) TypeTagClass else WeakTypeTagClass
+        val tagTp =  if (pre == NoType) TypeRef(ApiUniverseClass.toTypeConstructor, tagSym, List(tp)) else singleType(pre, pre member tagSym.name)
+        val taggedTp = appliedType(tagTp, List(tp))
+        resolveTag(pos, taggedTp, allowMaterialization)
+      }
   }
 }
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