Does not compile on 2.11 otherwise:

```scala
[error] .../pickling/core/src/main/scala/pickling/FastTags.scala:58: macro implementation has incompatible shape:
[error]  required: (c: scala.reflect.macros.whitebox.Context)(key: c.Expr[String]): c.Expr[scala.pickling.FastTypeTag[_$7]] forSome { type _$7 }
[error]  or      : (c: scala.reflect.macros.whitebox.Context)(key: c.Tree): c.Tree[]
[error]  found   : (c: reflect.macros.Context)(key: c.Expr[String]): c.Expr[scala.pickling.FastTypeTag[_]]
[error] type mismatch for return type: c.Expr[scala.pickling.FastTypeTag[_]] does not conform to c.Expr[scala.pickling.FastTypeTag[_$7]] forSome { type _$7 }
[error]   def apply(key: String): FastTypeTag[_] = macro Compat.FastTypeTagMacros_apply
[error]                                                         ^
[error] one error found
```