52385.jl

julia> using StructArrays

julia> begin
       g(x::StructArray{<:Tuple{String}}) = nothing
       g(x::StructArray{<:Tuple{Integer}}) = nothing
       function f(x)
           # ok, yes, this line is fairly contrived, but this is a short-hand for
           # building a partial type-instability -- something that happens frequently
           z1 = Core.compilerbarrier(:type, x)::StructArray{<:Tuple{Nothing}}
           z2 = g(z1)
           nothing === z2, z2
       end
       end;

julia> f(StructArray((Union{}[],)))
ERROR: MethodError: g(::StructVector{Tuple{Union{}}, Tuple{Vector{Union{}}}, Int64}) is ambiguous.

Candidates:
  g(x::StructArray{<:Tuple{String}})
    @ Main REPL[2]:2
  g(x::StructArray{<:Tuple{Integer}})
    @ Main REPL[2]:3

Possible fix, define
  g(::StructArray{Union{}})