A hint is that Julia itself can print the arguments. So step into show with the debugger:
julia> using Debugger
julia> function f(x,y::Integer,z=3) # just for fun, add a type parameter
p = x+y*z
return p
end
f (generic function with 4 methods)
julia> m = first(methods(f))
f(x, y::Integer) in Main at REPL[5]:2
julia> @enter show(m)
In show(x) at show.jl:325
>325 show(x) = show(stdout::IO, x)
About to run: (typeassert)(Base.TTY(RawFD(0x0000000d) open, 0 bytes waiting), IO)
1|debug> se
In show(x) at show.jl:325
>325 show(x) = show(stdout::IO, x)
About to run: (show)(Base.TTY(RawFD(0x0000000d) open, 0 bytes waiting), f(x, y::Integer) in Main at REPL[5]:2)
1|debug> s
In #show#393(kwtype, , io, m) at methodshow.jl:159
158 function show(io::IO, m::Method; kwtype::Union{DataType, Nothing}=nothing)
>159 tv, decls, file, line = arg_decl_parts(m)
160 sig = unwrap_unionall(m.sig)
161 ft0 = sig.parameters[1]
162 ft = unwrap_unionall(ft0)
163 d1 = decls[1]
About to run: Core.NewvarNode(:(_15))
1|debug> n
In #show#393(kwtype, , io, m) at methodshow.jl:159
158 function show(io::IO, m::Method; kwtype::Union{DataType, Nothing}=nothing)
159 tv, decls, file, line = arg_decl_parts(m)
>160 sig = unwrap_unionall(m.sig)
161 ft0 = sig.parameters[1]
162 ft = unwrap_unionall(ft0)
163 d1 = decls[1]
164 if sig === Tuple
About to run: (getproperty)(f(x, y::Integer) in Main at REPL[5]:2, :sig)
1|julia> decls
3-element Array{Tuple{String,String},1}:
("", "typeof(f)")
("x", "")
("y", "Integer")