julia> collect(Iterators.flatten((1, )))
ERROR: MethodError: no method matching promote_typejoin(::Type{Int64})
The function `promote_typejoin` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  promote_typejoin(::Any, ::Any)
   @ Base promotion.jl:174

Stacktrace:
 [1] eltype(::Type{Base.Iterators.Flatten{Tuple{Int64}}})
   @ Base.Iterators ./iterators.jl:1205
 [2] eltype(x::Base.Iterators.Flatten{Tuple{Int64}})
   @ Base ./abstractarray.jl:241
 [3] _collect(cont::UnitRange{Int64}, itr::Base.Iterators.Flatten{Tuple{Int64}}, ::Base.HasEltype, isz::Base.HasLength)
   @ Base ./array.jl:733
 [4] collect(itr::Base.Iterators.Flatten{Tuple{Int64}})
   @ Base ./array.jl:727
 [5] top-level scope
   @ REPL[2]:1

julia> versioninfo()
Julia Version 1.12.0-DEV.1734
Commit e0656ac017 (2024-12-08 21:48 UTC)
Platform Info:
  OS: macOS (arm64-apple-darwin24.1.0)
  CPU: 11 × Apple M3 Pro
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, apple-m3)
Threads: 1 default, 0 interactive, 1 GC (on 5 virtual cores)

julia> collect(Iterators.flatten((1, ))) 
1-element Vector{Int64}:
 1

julia> versioninfo() # also works on 1.10.7
Julia Version 1.11.2
Commit 5e9a32e7af2 (2024-12-01 20:02 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 11 × Apple M3 Pro
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m2)
Threads: 1 default, 0 interactive, 1 GC (on 5 virtual cores)

julia> collect(Iterators.flatten((1, (1, ))))
2-element Vector{Int64}:
 1
 1

julia> collect(Iterators.flatten((1, (1, ), 1)))
3-element Vector{Int64}:
 1
 1
 1

julia> collect(Iterators.flatten((1, 1, 1)))
ERROR: MethodError: no method matching promote_typejoin(::Type{Int64}, ::Type{Int64}, ::Type{Int64})
The function `promote_typejoin` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  promote_typejoin(::Any, ::Any)

julia> a = 1
1

julia> collect(Iterators.flatten((a, )))
ERROR: MethodError: no method matching promote_typejoin(::Type{Int64})
The function `promote_typejoin` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  promote_typejoin(::Any, ::Any)
   @ Base promotion.jl:174

Stacktrace:
 [1] eltype(::Type{Base.Iterators.Flatten{Tuple{Int64}}})
   @ Base.Iterators ./iterators.jl:1205
 [2] eltype(x::Base.Iterators.Flatten{Tuple{Int64}})
   @ Base ./abstractarray.jl:241
 [3] _collect(cont::UnitRange{Int64}, itr::Base.Iterators.Flatten{Tuple{Int64}}, ::Base.HasEltype, isz::Base.HasLength)
   @ Base ./array.jl:733
 [4] collect(itr::Base.Iterators.Flatten{Tuple{Int64}})
   @ Base ./array.jl:727
 [5] top-level scope
   @ REPL[11]:1

julia> collect(Iterators.flatten((a, a)))
2-element Vector{Int64}:
 1
 1

julia> collect(Iterators.flatten((a, a, a)))
ERROR: MethodError: no method matching promote_typejoin(::Type{Int64}, ::Type{Int64}, ::Type{Int64})
The function `promote_typejoin` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  promote_typejoin(::Any, ::Any)
   @ Base promotion.jl:174

Stacktrace:
 [1] eltype(::Type{Base.Iterators.Flatten{Tuple{Int64, Int64, Int64}}})
   @ Base.Iterators ./iterators.jl:1205
 [2] eltype(x::Base.Iterators.Flatten{Tuple{Int64, Int64, Int64}})
   @ Base ./abstractarray.jl:241
 [3] _collect(cont::UnitRange{Int64}, itr::Base.Iterators.Flatten{Tuple{…}}, ::Base.HasEltype, isz::Base.HasLength)
   @ Base ./array.jl:733
 [4] collect(itr::Base.Iterators.Flatten{Tuple{Int64, Int64, Int64}})
   @ Base ./array.jl:727
 [5] top-level scope
   @ REPL[13]:1
Some type information was truncated. Use `show(err)` to see complete types.

MethodError: no method matching promote_typejoin(::Type{Symbol}, ::Type{Symbol}, ::Type{Symbol}, ::Type{Symbol})
  The function `promote_typejoin` exists, but no method is defined for this combination of argument types.
  
  Closest candidates are:
    promote_typejoin(::Any, ::Any)
     @ Base promotion.jl:174
  
  Stacktrace:
    [1] eltype(::Type{Base.Iterators.Flatten{NTuple{4, Tuple{Symbol}}}})
      @ Base.Iterators ./iterators.jl:1205
    [2] eltype(x::Base.Iterators.Flatten{NTuple{4, Tuple{Symbol}}})
      @ Base ./abstractarray.jl:241
    [3] _collect(cont::UnitRange{Int64}, itr::Base.Iterators.Flatten{NTuple{4, Tuple{Symbol}}}, ::Base.HasEltype, isz::Base.HasLength)
      @ Base ./array.jl:733
    [4] collect(itr::Base.Iterators.Flatten{NTuple{4, Tuple{Symbol}}})

julia> collect(Iterators.flatten((:a, :b, :c, :d)))
ERROR: MethodError: no method matching promote_typejoin(::Type{Any}, ::Type{Any}, ::Type{Any}, ::Type{Any})
The function `promote_typejoin` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  promote_typejoin(::Any, ::Any)
   @ Base promotion.jl:174

Stacktrace:
 [1] eltype(::Type{Base.Iterators.Flatten{NTuple{4, Symbol}}})
   @ Base.Iterators ./iterators.jl:1205
 [2] eltype(x::Base.Iterators.Flatten{NTuple{4, Symbol}})
   @ Base ./abstractarray.jl:241
 [3] _collect(cont::UnitRange{Int64}, itr::Base.Iterators.Flatten{NTuple{…}}, ::Base.HasEltype, isz::Base.SizeUnknown)
   @ Base ./array.jl:737
 [4] collect(itr::Base.Iterators.Flatten{NTuple{4, Symbol}})