This morning I was sent the following warning by hex

A new Hex version is available (0.17.1 < 0.17.3), please update with `mix local.hex`

I have updated to the latest by running

 mix do local.rebar --force, local.hex --force, deps.get

The error I now see is related to some missing function.

Resolving Hex dependencies…
Dependency resolution completed:
unicode_util_compat 0.2.0
uuid 1.1.7

• Getting ace (Hex package)
  ** (UndefinedFunctionError) function Hex.unpack_tar!/2 is undefined or private
  (hex) Hex.unpack_tar!(“/opt/app/.hex/packages/hexpm/ace-0.15.0.tar”, “/cache/deps/ace”)
  (hex) lib/hex/scm.ex:129: Hex.SCM.update/1
  (hex) lib/hex/scm.ex:163: Hex.SCM.checkout/1
  (mix) lib/mix/dep/fetcher.ex:64: Mix.Dep.Fetcher.do_fetch/3
  (mix) lib/mix/dep/converger.ex:180: Mix.Dep.Converger.all/9
  (mix) lib/mix/dep/converger.ex:119: Mix.Dep.Converger.all/7
  (mix) lib/mix/dep/converger.ex:104: Mix.Dep.Converger.all/4
  (mix) lib/mix/dep/converger.ex:50: Mix.Dep.Converger.converge/4

The version information from elixir -v is

Erlang/OTP 20 [erts-9.0.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]

Elixir 1.5.1

Where is the Hex module defined? I looked for a hex package called hex and in the mix source. With that I could try and debug further.