When looking at a macro implementation, the compiler looks at parameters to recognize, among other less semantic sanity-checks, that the only implicit parameter is a WeakTypeTag.

This is done in scala.reflect.macros.compiler's Validators, through the building of macroImplSig. The exact place of comparison is in scala.reflect.macros.util.Helpers' transformTypeTagEvidenceParams.
If it fails you get a MacroImplNonTagImplicitParameters.

But from what I understand, the WeakTypeTagClass being compared against is (scala-)version-specific, so that when the 2.11 compiler works in -Xsource:2.10 mode, against a macro implementation coming from a 2.10 classfile, the WeakTypeTagClass it should try to find an instance of is not the one it has loaded, but the one in the 2.10 scala-reflect jar it (hopefully has on classpath).

So the symptom is, when compiling the play framework on 2.10 with -Xsource:2.10 and -Ymacro-expand:none:

/home/huitseeker/Scala/playframework/framework/src/play-json/src/main/scala/play/api/libs/json/Json.scala:194: error: macro implementations cannot have implicit parameters other than WeakTypeTag evidences
  def reads[A] = macro JsMacroImpl.readsImpl[A]
                                   ^
/home/huitseeker/Scala/playframework/framework/src/play-json/src/main/scala/play/api/libs/json/Json.scala:214: error: macro implementations cannot have implicit parameters other than WeakTypeTag evidences
  def writes[A] = macro JsMacroImpl.writesImpl[A]
                                    ^
/home/huitseeker/Scala/playframework/framework/src/play-json/src/main/scala/play/api/libs/json/Json.scala:234: error: macro implementations cannot have implicit parameters other than WeakTypeTag evidences
  def format[A] = macro JsMacroImpl.formatImpl[A]
                                    ^
three errors found

The relevant signatures from JsMacroImpl are fine:

  def readsImpl[A: c.WeakTypeTag](c: Context): c.Expr[Reads[A]] = {
  def writesImpl[A: c.WeakTypeTag](c: Context): c.Expr[Writes[A]] = {
  def formatImpl[A: c.WeakTypeTag](c: Context): c.Expr[Format[A]] = {

Is there a way for transformTag to dereference the class and manually fetch that 2.10 WeakTypeTagClass from classpath to compare against ?