Empty case is valid but unreachable

[zanchey]

The documentation says this is valid syntax, and indeed it is:

switch $foo
    case
        echo Hello
end

However, the case without an argument is unreachable. A parameter to switch that expands to nothing is equivalent to the empty string (per https://github.com/fish-shell/fish-shell/blob/master/src/parse_execution.cpp#L534-L539).

Should case without an argument be a default (equivalent to case '*') or an error? Or should it be left alone?

[faho]

I'd probably make a literal empty case error out and leave the commandline incomplete. As it stands you can type the example interactively, and it looks like it works.

Now, case with an empty expansion is another case (hahaha), but there it's probably expected that it won't match. E.g.

set -l cases # empty

switch $foo
    case foo
        echo foo
    case $cases
        # unreachable!
end

This seems to be expected and useful (because the variable also could be non-empty), while a literal empty case is just nothing.

This follows switch, which also needs an argument - it's a syntax error otherwise!