New ambiguous implicits error in Scala 2.11.4 (works in 2.11.2) with macros

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If you compile a.scala, then b.scala with 2.11.4, then you get the ambiguous implicits error shown beneath. For the problem to be apparent, a.scala must be compiled with 2.11.4, but b.scala can be compiled with either 2.11.2 or 2.11.4 (i.e. I'd infer that the problem is with the binaries produced by 2.11.4).

The most interesting thing about it is that the method foo and the macro seem to be completely unrelated. The error still occurs if foo gets moved to a separate object.

You can also see the problem in the REPL.

a.scala:

import language.experimental.macros
import scala.reflect.macros._
object X {
  def foo[D] = new Array[Any](0).map(_.asInstanceOf[D])
  def extractor: Any = macro extractorMacro
  def extractorMacro(c: blackbox.Context): c.Expr[Any] =
    c.universe.reify(new { def something(data: Any) = ??? })
}

b.scala:

object Test { X.extractor }

output:

> scalac a.scala && scalac b.scala
b.scala:1: error: ambiguous implicit values:
 both value StringCanBuildFrom in object Predef of type => scala.collection.generic.CanBuildFrom[String,Char,String]
 and method $conforms in object Predef of type [A]=> <:<[A,A]
 match expected type T
object Test { X.extractor }
                ^
one error found

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Imported From: https://issues.scala-lang.org/browse/SI-8947?orig=1
Reporter: @propensive
Affected Versions: 2.11.4

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@paulp said:
Roll those non-determinism dice and say "baby needs a new pair of shoes."

Here's a way to get the same error with one little file, on 2.11.2 or 2.11.4, which may serve to illustrate what's going on. Then again, it may not.

object X { implicitly }

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@retronym said:
My bet is that the non determinism once again comes from the MRU-cache like sorting performed in implicit search. I'll take a look.

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@retronym said:
Interesting, we now typecheck a naked call to implicitly

https://gist.github.com/retronym/60b1abfecb69970bf543

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@retronym said (edited on Oct 30, 2014 2:11:17 AM UTC):
We've got a new form of cross talk: tree attachments on EmptyTree!

scala/scala#4083

@propensive I've listed a few workarounds in the test case in the pull request.

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@propensive said:
Thanks for the quick turnaround, Jason!