Disclaimer: I don't personally use Windows and I inferred this behavior only through appveyor. So maybe I'm missing some points here.

Anyway, how do you update PATH to be used for setenv(command::Cmd, env) on Windows? For example, if I do

env_var = copy(ENV)
env_var["PATH"] = "some_new_path" * ';' * env_var["PATH"]
setenv(command::Cmd, env_var)

it doesn't work since the original PATH is in env_var["Path"] not env_var["PATH"]. I had to implement something like this (JuliaPy/Conda.jl#121 (comment)):

env_var = copy(ENV)
all_keys = collect(keys(env_var))
ikey = findfirst(x -> uppercase(x) == "PATH", all_keys)
has_key = ikey !== nothing
path_key = has_key ? all_keys[ikey] : "PATH"
if has_key
    env_var[path_key] = extra_path * path_sep * env_var[path_key]
else
    env_var[path_key] = extra_path
end

I tried empty!(ENV); merge!(ENV, env_var) but apparently empty!(ENV) is not implemented (at least on Linux). Also, it would have emitted bunch of syscalls so maybe not a good idea in the first place.

So, I thought maybe Base needs to have a case-insensitive (in-memory) dictionary and copy(ENV) on Windows should return it. It also facilitates cross-platform testing.

cc: @stevengj