You can use invoke(show, Tuple{IO,Any}, stdout, x) to invoke the default show(::IO, ::Any) method instead of any overloaded definition for x:
julia> invoke(show, Tuple{IO,Any}, stdout, pi)
Irrational{:π}()
Alternatively, you can call Base.show_default(io, x) (which happens to be what show(::IO, ::Any) calls), but invoke is the more general technique to call a less-specific method.