error example does not work as expected

[emilienkofman]

Describe the bug
The example provided here does not seem to work as expected. If PORT is not defined it correctly prints

=== RUN   TestExample
$PORT is not set
--- PASS: TestExample (0.00s)
PASS

However if PORT is defined it fails

Failed: must provide constructor function, got &{127.0.0.1:8080 <nil> false <nil> 0s 0s 0s 0s 0 map[] <nil> <nil> <nil> <nil> {{} 0} {{} 0} {{{} 0} {0 0}} <nil> {0 0} map[] map[] [] {{} {{} {} 0} 0}} (type *http.Server)

To Reproduce
Drop the example in a go unit test and add t.SetEnv("PORT", 8080)

Expected behavior
I guess the call to fx.Provide must be replaced with

return fx.Provide(
    func() *http.Server {
        return &http.Server{Addr: fmt.Sprintf("127.0.0.1:%s", port)}
    }
)

which actually reaches fx.Invoke ListenAndServe

Additional context
Stumbled accross this because I'm trying to figure out a good way to handle errors when reading configuration (from env variables but that could be in a file as well). Generally speaking, is it a good idea to delay error handling in that case? I understand that this fx.Error allows to delay config read error after the call to fx.New thus maybe allows to execute cleanup code if needed.

edit: regarding that Provide error, would this make any sense to have a hook that triggers on Provide errors (e.g. before app.Start here)