`map` throws type assertion error

[MrVPlusOne]

Hi,

(I'm opening this as a separate issue for easy tracking and to make sure people see this.)

I just noticed that the type inference related changes newly introduced in #42046 breaks my existing code. A minimal example:

map(1:4) do i
    a = (x=i, y=(i==1 ? 1 : "a"))
    b = 3
    (a, b)
end

throws the following error

ERROR: LoadError: TypeError: in typeassert, expected AbstractVector{<:Tuple{NamedTuple{(:x, :y), _A} where _A<:Tuple{Int64, Union{Int64, String}}, Int64}}, got a value of type Vector{Tuple{NamedTuple{(:x, :y)}, Int64}}
Stacktrace:
 [1] _collect(c::UnitRange{Int64}, itr::Base.Generator{UnitRange{Int64}, Main.SEDL.var"#636#637"}, #unused#::Base.EltypeUnknown, isz::Base.HasShape{1})
   @ Base ./array.jl:754
 [2] collect_similar(cont::UnitRange{Int64}, itr::Base.Generator{UnitRange{Int64}, Main.SEDL.var"#636#637"})
   @ Base ./array.jl:653
 [3] map(f::Function, A::UnitRange{Int64})
   @ Base ./abstractarray.jl:2849
 [4] top-level scope
   @ ~/.julia/dev/SEDL/scripts/hovercraft_example.jl:63

I just upgraded to 1.7.0-rc3 from rc1 this morning and immediately ran into this. Looks like the computed type annotation is more precise but is not compatible with the actual returned collection type.

[nalimilan]

Indeed this has been introduced by #42046. This is due to promote_typejoin_union, which returns a too precise type for NamedTuple elements:

julia> Base.promote_typejoin_union(NamedTuple{(:x, :y), Tuple{Int64, Union{Int64, String}}})
NamedTuple{(:x, :y), Tuple{Int64, Union{Int64, String}}}

# But map and collect use rather
julia> Base.promote_typejoin(NamedTuple{(:x, :y), Tuple{Int64, Int64}}, NamedTuple{(:x, :y), Tuple{Int64, String}})
NamedTuple{(:x, :y)}

promote_typejoin_union should check recursively whether the passed type contains any Unions parameters, and if so call itself on them.

Here's another example to trigger the problem:

julia> itr = (((x=i, y=(i==1 ? 1 : "a")), 3) for (i, _) in enumerate(1:4))
Base.Generator{Base.Iterators.Enumerate{UnitRange{Int64}}, var"#8#9"}(var"#8#9"(), enumerate(1:4))

julia> Base.promote_typejoin_union(Core.Compiler.return_type(Base._iterator_upper_bound, Tuple{typeof(itr)}))
Tuple{NamedTuple{(:x, :y), _A} where _A<:Tuple{Int64, Union{Int64, String}}, Int64}

[nalimilan]

I've made a first attempt at a fix at #43120. Unfortunately it's probably not correct, but let's hear what @vtjnash says.