Optional or named arguments

base/show.jl defines at line 2218 at least in v1.6 the function function show_tuple_as_call(io::IO, name::Symbol, sig::Type, demangle=false, kwargs=nothing, argnames=nothing, qualified=false)

I’m able to invoke it Base.show_tuple_as_call(stdout, timings.mi_info.mi.def.name, timings.mi_info.mi.specTypes) but not at Base.show_tuple_as_call(stdout, timings.mi_info.mi.def.name, timings.mi_info.mi.specTypes; demangle = true). What is the correct syntax to set named arguments?

Those are not keyword arguments but optional positional arguments. If you want a keyword argument, you need function f(x; keyword="default") in the definition (note the semicolon).