Bug in dispatching on a union containing a UnionAll type

[MasonProtter]

MWE:

julia> g(::Ref{Union{Int, Other}}) where {Other} = Other
g (generic function with 1 method)

julia> g(Ref{Union{Int, Float64}}(1))
Float64

julia> g(Ref{Union{Int, Array}}(1))
ERROR: MethodError: no method matching g(::Base.RefValue{Union{Int64, Array}})
Closest candidates are:
  g(::Ref{Union{Int64, Other}}) where Other at REPL[3]:1
Stacktrace:
 [1] top-level scope at REPL[5]:1

julia> versioninfo()
Julia Version 1.5.2
Commit 539f3ce943* (2020-09-23 23:17 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: AMD Ryzen 5 2600 Six-Core Processor
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, znver1)
Environment:
  JULIA_NUM_THREADS = 6

Seems like a bug with UnionAll to me.

Originally posted by @MasonProtter in #37790 (comment)

[MasonProtter]

In #37790 (comment), @vtjnash says that it's desirable to make it so that concrete types behave like UnionAll types here rather than the other way around. To me, this sounds bad, but can you explain a bit about why?

[JeffBezanson]

Duplicate of #26135