Fix broadcast error when eltype is inconsistent with getindex (#39185)

* Fix broadcast error when eltype is inconsistent with getindex

In that case we can infer the return type as `Union{}`, which triggers
a type assertion error. This used to work in Julia 1.5.

* whitespace

Co-authored-by: Simeon Schaub <simeondavidschaub99@gmail.com>

* Update test/broadcast.jl

Co-authored-by: Matt Bauman <mbauman@juliacomputing.com>

Co-authored-by: Matt Bauman <mbauman@juliacomputing.com>
Co-authored-by: Simeon Schaub <simeondavidschaub99@gmail.com>

base/broadcast.jl

@@ -924,7 +924,7 @@ const NonleafHandlingStyles = Union{DefaultArrayStyle,ArrayConflict}
     # Now handle the remaining values
     # The typeassert gives inference a helping hand on the element type and dimensionality
     # (work-around for #28382)
-    ElType′ = ElType <: Type ? Type : ElType
+    ElType′ = ElType === Union{} ? Any : ElType <: Type ? Type : ElType
     RT = dest isa AbstractArray ? AbstractArray{<:ElType′, ndims(dest)} : Any
     return copyto_nonleaf!(dest, bc′, iter, state, 1)::RT
 end

test/broadcast.jl

@@ -990,3 +990,15 @@ p0 = copy(p)
     @test isequal(identity.(Vector{<:Union{Int, Missing}}[[1, 2],[missing, 1]]),
                   [[1, 2],[missing, 1]])
 end
+
+@testset "Issue #28382: eltype inconsistent with getindex" begin
+    struct Cyclotomic <: Number
+    end
+
+    Base.eltype(::Type{<:Cyclotomic}) = Tuple{Int,Int}
+
+    Base.:*(c::T, x::Cyclotomic) where {T<:Real} = [1, 2]
+    Base.:*(x::Cyclotomic, c::T) where {T<:Real} = [1, 2]
+
+    @test Cyclotomic() .* [2, 3] == [[1, 2], [1, 2]]
+end