zig/test/cases/switch_prong_err_enum.zig

const assert = @import("std").debug.assert;

var read_count: u64 = 0;

fn readOnce() error!u64 {
    read_count += 1;
    return read_count;
}

const FormValue = union(enum) {
    Address: u64,
    Other: bool,
};

fn doThing(form_id: u64) error!FormValue {
    return switch (form_id) {
        17 => FormValue {
            .Address = try readOnce(),
        },
        else => error.InvalidDebugInfo,
    };
}

test "switch prong returns error enum" {
    switch (doThing(17) catch unreachable) {
        FormValue.Address => |payload| {
            assert(payload == 1);
        },
        else => unreachable,
    }
    assert(read_count == 1);
}