cp: --reflink with no argument should be assumed --reflink=always

[jfinkels]

The --reflink command line option to cp takes an optional argument (always, auto, or never, with always being the default) but it can be used without an argument. uutils cp does not allow this type of usage, but should

GNU cp v8.30:

$ touch f && cp --reflink f g
cp: failed to clone 'g' from 'f': Operation not supported

uutils cp:

$ touch f && ./target/debug/cp --reflink f g
./target/debug/cp: invalid argument 'f' for 'reflink'

This is similar to an issue with mktemp --tmpdir, described here:

coreutils/src/uu/mktemp/src/mktemp.rs

Lines 172 to 178 in 4b517a3

	// Special case to work around a limitation of `clap`; see
	// `is_tmpdir_argument_actually_the_template()` for more
	// information.
	//
	// Fixed in clap 3
	// See https://github.com/clap-rs/clap/pull/1587
	let (tmpdir, template) = if is_tmpdir_argument_actually_the_template(matches) {

[sssemil]

Can confirm. Working on this.

[sssemil]

Hm, so I can add .default_missing_value("auto") but then it still tries to take the file name as an argument. But I can also then set .require_equals(true), however it will fail if nothing is specified because = is required. It looks like there's no softer version of require_equals in clap.

[tertsdiepraam]

Maybe this can be of help? This is how we did the --color argument for ls.

coreutils/src/uu/ls/src/ls.rs

Lines 1428 to 1436 in 043c009

	Arg::new(options::COLOR)
	.long(options::COLOR)
	.help("Color output based on file type.")
	.takes_value(true)
	.value_parser([
	"always", "yes", "force", "auto", "tty", "if-tty", "never", "no", "none",
	])
	.require_equals(true)
	.min_values(0),

[sssemil]

@tertsdiepraam that works, thanks! Will make a PR now.