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Before 2.10 we had a notion of ClassManifest that could be used to retain
erasures of abstract types (type parameters, abstract type members) for
being used at runtime.
With the advent of ClassManifest (and its subtype Manifest)
it became possible to write:
def mkGenericArray[T: Manifest] = Array[T]()
When compiling array instantiation, scalac would use a ClassManifest
implicit parameter from scope (in this case, provided by a context bound)
to remember Ts that have been passed to invoke mkGenericArray and
use that information to instantiate arrays at runtime (via Java reflection).
When redesigning manifests into what is now known as type tags, we decided
to explore a notion of ArrayTags that would stand for abstract and pure array
creators. Sure, ClassManifests were perfectly fine for this job, but they did
too much - technically speaking, one doesn't necessarily need a java.lang.Class
to create an array. Depending on a platform, e.g. within JavaScript runtime,
one would want to use a different mechanism.
As tempting as this idea was, it has also proven to be problematic.
First, it created an extra abstraction inside the compiler. Along with class tags
and type tags, we had a third flavor of tags - array tags. This has threaded the
additional complexity though implicits and typers.
Second, consequently, when redesigning tags multiple times over the course of
Scala 2.10.0 development, we had to carry this extra abstraction with us, which
exacerbated the overall feeling towards array tags.
Finally, array tags didn't fit into the naming scheme we had for tags.
Both class tags and type tags sound logical, because, they are descriptors for
the things they are supposed to tag, according to their names.
However array tags are the odd ones, because they don't actually tag any arrays.
As funny as it might sound, the naming problem was the last straw
that made us do away with the array tags. Hence this commit.
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