else prong required when switching on pointer type

[digital roost]

Hi, I'm learning zig and I've found a confusing error when doing a switch on a pointer to a tagged union.

I need to access the union through a pointer because sometimes I need to mutate one of the inner fields of the union and other times I want to change to a different tag.

I've constructed a somewhat non-sensical example:

const Foo = union(enum) {
    a: u8,
    b,
};

pub fn main() !void {
    var foo: Foo = .b;

    while (true) {
        switch (&foo) {
            .a => |a| {
                a.* += 1;
            },
            .b => {
                foo = Foo{ .a = 0 };
            },
        }
    }
}

And then when running:

$ zig run wtf.zig
wtf.zig:10:9: error: else prong required when switching on type '*wtf.Foo'

shell returned 1

Why do I need an else branch on the switch? Is this avoidable?

[fleet viper]

when you need to switch on a pointer type, just do switch(ptr.*) { ...

[idle herald]

an else branch is required because youre switching on the value of the pointer, youre comparing address it contains, not the variant of what its pointing to

and your switch doesn’t have a prong for every single possible address, so it needs an else :P