String concatenation is done very early

gistfile1.txt

String concatenation happens before the tree is passed to your macro, 
so you get passed a Literal(Constant("ab"))

scala> reify(() => "a" + "b")
res14: res0.universe.Expr[() => String] = Expr[() => java.lang.String]((() => "ab"))


This is not the case with other String ops. 
I guess + is special because it's a Java operator, not a method on StringLike.

scala> reify(() => "a" * 5)
res15: res0.universe.Expr[() => String] = Expr[() => String]((() => Predef.augmentString("a").$times(5)))