Match semantics re. pattern order

[alimpfard]

Following from #1183 (comment)

Currently match patterns do not care about the order, as the patterns cannot be overlapping (aside from else, which is currently defined as "all cases not mentioned in the match"), so pattern order is ignored in enum matches.
Conversely, pattern order is significant in value matches, where we go down the list of values, checking for equality.

The first one allows matches like

match foo {
    else => x
    A => y
}

to yield y for A, which is dissimilar to other languages.

[robryanx]

Was having a look at the c/c++ switch statement which does allow the default to be anywhere.
So similar to that it would make some sense to change the value matches.
We could also go the other way and enforce else being the last match to be more like if else.

[alimpfard]

we can't really change the semantics of value matches unless we explicitly check for no overlaps, consider this:

let x = 1
match 1 {
    0..10 => "foo"
    1..3 => "foo, but not quite"
    (x) => "why"
    else => "else"
}

this is valid, none of the match arms are the same constant (though it might make sense to disallow), but there's no real way to do a full overlap check on these.

[robryanx]

That seems okay, it can be that the order matters except for the else case that only matches if none of the other cases do.

[maddanio]

else has to always be treated as the last branch, wether enforced to be written so or not. nothing can come after it, so not ordering it last does not make any sense