x.json2: encoder omits `_type` discriminator field for sumtype values, breaking round-trip

[enghitalo]

Describe the bug

When encoding a sumtype value with x.json2, the produced JSON does not include the discriminator field _type that the existing json (cJSON-based) encoder always adds. This breaks round-trip encode → decode and breaks compatibility with any consumer expecting the _type discriminator that the legacy json module emits and consumes.

x.json2.encode_sumtype simply forwards the inner variant to encode_value without writing any discriminator key.

V code

import x.json2

type Animal = Cat | Dog

struct Cat {
	cat_name string
}

struct Dog {
	dog_name string
}

fn main() {
	out := json2.encode(Animal(Dog{ dog_name: 'Caramelo' }))
	println(out)
}

Reproduction Steps

import x.json2

type Animal = Cat | Dog

struct Cat {
	cat_name string
}

struct Dog {
	dog_name string
}

fn test_sumtype_emits_type_discriminator() {
	out := json2.encode(Animal(Dog{ dog_name: 'Caramelo' }))
	assert out == '{"dog_name":"Caramelo","_type":"Dog"}'
}

Expected Behavior

PASS test_sumtype_emits_type_discriminator

Current Behavior

> assert out == '{"dog_name":"Caramelo","_type":"Dog"}'
  Left value (len: 23): `{"dog_name":"Caramelo"}`
  Right value (len: 37): `{"dog_name":"Caramelo","_type":"Dog"}`

Possible Solution

In the encoder (encode_sumtype and the corresponding object writer used for struct variants), after writing the inner struct fields:

1. Detect that the value's static type is a sumtype variant of a struct.
2. Before closing the object }, append ,"_type":"<VariantTypeName>" (use T.name of the matched variant).
3. For non-struct variants (int, string, bool, time.Time), use the same wrapping the cJSON encoder uses — time.Time becomes {"_type":"Time","value":<unix>}, primitives stay bare (cJSON keeps them bare too).
4. Mirror this in the decoder: when decoding a sumtype struct variant, prefer the _type key when present and fall back to structural matching only if it is missing (the []StringOrComplexValue test depends on the structural fallback that already exists in the decoder for the no-_type case).

V's legacy C codegen for the json module already does this when expanding sumtype encoders — that logic should be ported to x.json2.

Additional Information/Context

The same root cause makes the json_sumtype todo test under x.json2, the converted cJSON sumtype encode test, and the cJSON test_encode_sumtype_defined_ahead case all fail when they target x.json2.

V version

V 0.5.1 1b3385cc34ff783e793d1a26a8ec5be587c80fe0.40b3711

Environment details (OS name and version, etc.)

|V full version      |V 0.5.1 1b3385cc34ff783e793d1a26a8ec5be587c80fe0.40b3711
|:-------------------|:-------------------
|OS                  |linux, Ubuntu 24.04 LTS
|Processor           |16 cpus, 64bit, little endian, AMD Ryzen 7 5800H with Radeon Graphics
|Memory              |8.17GB/30.7GB
|                    |
|V executable        |/home/hitalo/Documents/v/v
|V last modified time|2026-04-18 09:18:00
|                    |
|V home dir          |OK, value: /home/hitalo/Documents/v
|VMODULES            |OK, value: /home/hitalo/.vmodules
|VTMP                |OK, value: /tmp/v_1000
|Current working dir |OK, value: /home/hitalo/Documents/v
|                    |
|Git version         |git version 2.43.0
|V git status        |0.5.1-1006-g40b3711b-dirty
|.git/config present |true
|                    |
|cc version          |cc (GCC) 14.2.0
|gcc version         |gcc (GCC) 14.2.0
|clang version       |Ubuntu clang version 18.1.3 (1)
|tcc version         |tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
|tcc git status      |thirdparty-linux-amd64 696c1d84
|emcc version        |emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.6 ()
|glibc version       |ldd (Ubuntu GLIBC 2.39-0ubuntu8.3) 2.39

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