[json] decoding struct with i32 member results in cgen error: json: i32 is not struct #21161

Describe the bug

x.json2 expects a type to be provided. When the type is a struct with an i32 field, the below error is output:

cgen error: json: i32 is not struct

Reproduction Steps

import json

pub struct StructA {
	kind string
	value string
}
pub struct StructB {
	kind string
	value i32
}

type StructSum = StructA | StructB

fn main() {
	// cgen error: json: i32 is not struct
	json.decode(StructSum, '{"kind": "Int32", value: 100}')!
}

Expected Behavior

json.decode successfully decodes the json into an instance of StructB

Current Behavior

cgen error: json: i32 is not struct

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.5 5da1fcc

Environment details (OS name and version, etc.)

V full version: V 0.4.5 4687f8c.5da1fcc
OS: windows, Microsoft Windows 11 Pro v22631 64-bit
Processor: 12 cpus, 64bit, little endian,

getwd: C:\dev\v\winmd
vexe: C:\dev_tools\v\v.exe
vexe mtime: 2024-04-01 17:34:15

vroot: OK, value: C:\dev_tools\v
VMODULES: OK, value: C:\Users\simen.vmodules
VTMP: OK, value: C:\Users\simen\AppData\Local\Temp\v_0

Git version: git version 2.40.0.windows.1
Git vroot status: weekly.2024.13-65-g5da1fcca
.git/config present: true

CC version: Error: 'cc' is not recognized as an internal or external command,
operable program or batch file.

thirdparty/tcc status: thirdparty-windows-amd64 a39eb79b