else value when switching on error set should have optional capture value which is subset

[andrewrk]

pub fn atomicSymLink(allocator: &Allocator, existing_path: []const u8, new_path: []const u8) !void {
    if (symLink(allocator, existing_path, new_path)) {
        return;
    } else |err| switch (err) {
        error.PathAlreadyExists => {},
        else => return err, // TODO zig should know this set does not include PathAlreadyExists
    }
    ...
}

It should not be possible for atomicSymLink to return error.PathAlreadyExists.

Here's my proposal:

pub fn atomicSymLink(allocator: &Allocator, existing_path: []const u8, new_path: []const u8) !void {
    if (symLink(allocator, existing_path, new_path)) {
        return;
    } else |err| switch (err) {
        error.PathAlreadyExists => {},
        else => |subset| return subset,
    }
    ...
}

In this example, subset has exactly the same value as err, except that it has a different type. The type is @typeOf(err) except the error set does not have any of the errors covered by the switch prongs above.

If the type of subset would be error{} then the else is dead code.

[andrewrk]

Also you should be able to do it with other switch items:

                (await (async self.channel.loop.bsdWaitKev(
                    @intCast(usize, fd), posix.EVFILT_VNODE, posix.NOTE_WRITE,
                ) catch unreachable)) catch |err| switch (err) {
                    error.EventNotFound => unreachable,
                    error.ProcessNotFound => unreachable,
                    error.AccessDenied, error.SystemResources => {
                        // TODO https://github.com/ziglang/zig/issues/769
                        const casted_err = @errSetCast(error{AccessDenied,SystemResources}, err);
                    },
                };

[andrewrk]

This now works for the else capture value. What's left to do before closing is supporting a capture value when there are multiple error cases specified (the example in #769 (comment))