Core.Compiler.return_type is inserting an Array UnionAll out of nowhere

[schlichtanders]

Hi Julia core team,

today I stumbled upon some magic breaking down.
Core.Compiler.return_type should be taken cautiously, I know, however it worked super well in many many cases for me. But this one.

struct One{T}
  value::T
end
struct Two{T}
  value::T
end
Base.map(f, x::One) = One(f(x.value))
Base.map(f, x::Two) = Two(f(x.value))

Core.Compiler.return_type(map, Tuple{typeof(string), Union{One{Int}, Two{T}} where T})

the expected outcome is

Union{One{String}, Two{_A} where _A}

however it returns

Union{One{String}, Array{T,N} where T where N, Two{_A} where _A}

as you see, somehow an Array UnionAll got inserted.

For sure, this has something todo with the quite special UnionAll construction Union{One{Int}, Two{T}} where T} where the UnionAll variable is only used for part of the inner Union.

---

EDIT

julia> versioninfo()
Julia Version 1.4.1
Commit 381693d3df* (2020-04-14 17:20 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin18.7.0)
  CPU: Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-8.0.1 (ORCJIT, skylake)