else => unreachable should be handled like catch unreachable when the type is an error set

[andrewrk]

test "else => unreachable hitting error" {
    foo() catch |e| switch (e) {
        error.NotThisOne => {},
        else => unreachable,
    };
}

const E = error{
    NotThisOne,
    OtherOne,
};

fn foo() E!void {
    return error.OtherOne;
}

output:

1/1 test "else => unreachable hitting error"...reached unreachable code
/home/andy/Downloads/zig/build/test.zig:4:17: 0x2055b6 in test "else => unreachable hitting error" (test)
        else => unreachable,
                ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:13:25: 0x228471 in std.special.main (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/Downloads/zig/lib/std/special/start.zig:204:37: 0x2272e5 in std.special.posixCallMainAndExit (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/special/start.zig:102:5: 0x22715f in std.special._start (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/Downloads/zig/build/zig-cache/o/4P7VSNxF-q2bqL786SGteb7Jk6o2zFfGEcF1BiRIoO8a0yEuWJi1uW37KiUFAk2X/test

Expected output: something like this:

1/1 test "else => unreachable hitting error"...attempt to unwrap error: OtherOne
/home/andy/Downloads/zig/build/test.zig:11:5: 0x2056f8 in foo (test)
    return error.OtherOne;
    ^
???:?:?: 0x20579c in ??? (???)


/home/andy/Downloads/zig/build/test.zig:4:13: 0x2055b2 in test "else => unreachable hitting error" (test)
    else => unreachable;
            ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:13:25: 0x228501 in std.special.main (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/Downloads/zig/lib/std/special/start.zig:204:37: 0x227375 in std.special.posixCallMainAndExit (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/special/start.zig:102:5: 0x2271ef in std.special._start (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/Downloads/zig/build/zig-cache/o/ESeTr9sZaQtjsYqR6CNXHF8ySsYnUhoGitQryIvDP54IW8FOmCiGHivusuRweVdD/test

[andrewrk]

Note: here is a workaround until this is fixed:

        else => (anyerror!void)(err) catch unreachable,

[Vexu]

Added in #12807