MWE for failing test for errorshow.jl or "Varargs, the undead of the Julia Type system"

[macd]

It turns out that the failure in errorshow.jl has to do with handling
Varargs. Due to JuliaLang/julia#30995, there is
a check in prepare_call in construct.jl that will punt if any of the args
is a Vararg. But that is the problem, there is not an easy way to test the
different forms of Vararg that come through.

The test is currently elseif any(x->isa(x, Type) && x <: Vararg, allargs)
however, one form of that comes through is x = Vararg{AbstractString,N} where {N}
and isa(x, Type) tests true but x <: Vararg is false. typeof(x) returns
a UnionALL. Another arg that comes through is Vararg{AbstractString,N}, that
is, there is no where clause?! I don't understand how that is possible.

I could not construct a test to get these two, and only these two, using type checking.
For the purposes of testing, I used the brute force approach of
elseif any(x->isa(x, Type) && occursin("Vararg", string(x)), allargs) The unit
tests for JuliaInterpreter run OK with this and the following MWE passes, but there should
be a better way to do that, so I did not prepare a PR.

using JuliaInterpreter
include("utils.jl")

module m
    method_c1(x::Float64, s::AbstractString...) = true
    buf = IOBuffer()
    me = Base.MethodError(method_c1,(1, 1, ""))
end

ex = quote
    Base.show_method_candidates(buf, me)
end

# TypeError: in Type, in parameter, expected Type, got Vararg{AbstractString,N} where N

frame = JuliaInterpreter.prepare_thunk(m, ex)
JuliaInterpreter.finish_and_return!(frame)