debug failed attempt of creating union enum in Zig

create_union_enum.zig

const std = @import("std");
const expect = std.testing.expect;

const cell = struct {
    v1: ?*Variant,
    v2: ?*Variant,
};

const Variant = union(enum) {
    int: i32,
    boolean: bool,
    cell: cell,

    // void can be omitted when inferring enum tag type.
    none,

    fn truthy(self: Variant) bool {
        return switch (self) {
            Variant.int => |x_int| x_int != 0,
            Variant.boolean => |x_bool| x_bool,
            Variant.cell => false,
            Variant.none => false,
        };
    }
};

test "union method" {
    var v1 = Variant{ .int = 1 };
    var v2 = Variant{ .boolean = false };

    try expect(v1.truthy());
    try expect(!v2.truthy());
}

test "create union" {
    const allocator = std.testing.allocator;

    var v1 = try allocator.create(Variant);
    defer allocator.destroy(v1);

    v1.* = .{ .int = 1 };
    try expect(v1.truthy());
}

test "create union" {
    const allocator = std.testing.allocator;

    var top = try allocator.create(Variant);
    defer allocator.destroy(top);
    top.* = .{ .cell = cell{ .v1 = null, .v2 = null } };

    var v1 = top;

    var i: i32 = 0;
    while (i < 3) : (i += 1) {
        std.log.warn("setting v1.cell.v1: v1={}", .{@ptrToInt(v1)});
        v1.cell.v1 = try allocator.create(Variant);
        v1.cell.v1.?.* = .{ .int = i + 1 };

        var v2 = try allocator.create(Variant);
        std.log.warn("v2 created: {}", .{@ptrToInt(v2)});
        defer allocator.destroy(v2);
        defer std.log.warn("v2 destroy: {}", .{@ptrToInt(v2)});
        v2.* = .{ .cell = cell{ .v1 = null, .v2 = null } };
        v1.cell.v2 = v2;
        v1 = v2;
    }

    std.log.warn("{}", .{top.cell.v2.?.cell.v1}); //
}

test.log

```
$ zig test create_union_enum.zig 
Test [3/3] test "create union"... [default] (warn): setting v1.cell.v1: v1=140619163426816
[default] (warn): v2 created: 140619163426880
[default] (warn): v2 destroy: 140619163426880
[default] (warn): setting v1.cell.v1: v1=140619163426880
thread 2444827 panic: access of inactive union field
/home/hnakamur/ghq/github.com/hnakamur/learn-zig/create_union_enum.zig:57:11: 0x20c85d in test "create union" (test)
        v1.cell.v1 = try allocator.create(Variant);
          ^
/home/hnakamur/zig/zig-0.10.0-dev.2989+4a28c1d5c/lib/test_runner.zig:79:28: 0x23496c in (root).main (test)
        } else test_fn.func();
                           ^
/home/hnakamur/zig/zig-0.10.0-dev.2989+4a28c1d5c/lib/std/start.zig:570:22: 0x20daf3 in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/home/hnakamur/zig/zig-0.10.0-dev.2989+4a28c1d5c/lib/std/start.zig:338:5: 0x20d8b2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
zig-cache/o/c2daa2bc1a64e662f8f659d4cf0783d5/test /home/hnakamur/zig/zig-0.10.0-dev.2989+4a28c1d5c/zig
```