1 files changed, 41 insertions, 0 deletions
diff --git a/test/files/pos/t8947/Macro_1.scala b/test/files/pos/t8947/Macro_1.scala
new file mode 100644
index 0000000000..4a5de3decb
--- /dev/null
+++ b/test/files/pos/t8947/Macro_1.scala
@@ -0,0 +1,41 @@
+import language.experimental.macros
+import scala.reflect.macros._
+import blackbox.Context
+
+object X {
+
+  def classTagOrNull[T](implicit t: reflect.ClassTag[T] = null) = t
+  // the failed search for ClassTag[T] does not issue a visible
+  // error as we fall back to the default argument. But, the
+  // macro engine things we have expanded the macro `materializeClassTag[D]()`
+  // to `EmptyTree`, and then attaches a backreference from the expansion
+  // to the expandee. This is the `MacroExpansionAttachment` tree attachment.
+  def foo[D] = classTagOrNull[D]
+
+  def extractor: Any = macro X.extractorMacro
+  def extractorMacro(c: Context): c.Expr[Any] = {
+    // Later, in reify, an unrelated use of `EmptyTree` in the AST representing
+    // the argument is now treated as a macro expansion which should be rolled
+    // back in the tree we reify! This ends up generating a call to `implicitly`
+    // which leads to an ambiguous error.
+    //
+    // Any macro call that expands to EmptyTree could have triggered this problem.
+    c.universe.reify(new { def something(data: Any) = ??? })
+  }
+
+  // Workarounds:
+  //
+  // 1. Use quasiquotes rather than `reify`. (But, beware to fully qualify all references, e.g. `_root_.scala.Predef.???`)
+  // 2. Avoid failed ClassTag lookups (e.g. in the original bug report, annotate the type argument to `map`)
+  // 3. In the macro implementation, just before calling the `reify` macro, you could call another macro
+  //
+  //      def prepareReify = macro prepareReifyImpl
+  //      def prepareReifyImpl(c: Context) = {
+  //        val symtab = c.universe.asInstanceOf[reflect.internal.SymbolTable]
+  //        symtab.EmptyTree.setAttachments(symtab.NoPosition)
+  //      }
+  //
+  //   To make this visible to the macro implementaiton, it will need to be compiled in an earlier stage,
+  //   e.g a separate SBT sub-project.
+
+}