Commits on Dec 15, 2020

1. Make Vararg not a DataType …

   Currently `Vararg` is a DataType, but is special cased in a
   bunch of places to give it special behavior (e.g. in subtyping
   and of course in tuple construction). However, unlike all other
   DataTypes, it cannot appear as a type parameter, which caused trouble in
   PR #38071. Having it be a DataType is a bit of a pun of convenience
   in the first place - it's a lot more similar to a tvar (which can
   be considered an implementation detail of UnionAll in the same way
   Vararg is an implementation detail of Tuple), which has its own
   non-type object. This PR does the same to Vararg, and moves it
   from being an abstract DataType with special cased behavior to
   its own custom type (called `Core.TypeofVararg`). There are a
   few small behavior differences, but they are mostly internal.
   In particular, we no longer have `Vararg <: Type` and Vararg
   objects no longer have the .parameters field. Also, things like
   `Vararg{T} where T` are technically illegal now since Vararg is
   not a type. However, since a lot of people are using that pattern,
   I've brought it back with a deprecation (which is of course off
   by default). The only things that's disallowed is `Vararg{N, N} where N`,
   but I haven't seen anybody use that.

   

   Keno committed Dec 15, 2020
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