The language allows circular type references

[gchumillas]

Describe the bug

Code: https://play.vlang.io/p/104cca0a8c

type Type6 = Type7 | int
type Type7 = Type6 | string

struct Foo {
	name Type7
}

Reproduction Steps

Run the code above

Expected Behavior

It shouldn't allow circular type references

Current Behavior

It allows circular type references

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.10 d06e11188b980439bb46dfdfe8548e8a536ad2a5

Environment details (OS name and version, etc.)

|V full version      |V 0.4.10 d06e11188b980439bb46dfdfe8548e8a536ad2a5
|:-------------------|:-------------------
|OS                  |linux, Debian GNU/Linux 12 (bookworm) (VM)
|Processor           |2 cpus, 64bit, little endian, Intel(R) Xeon(R) CPU E5-2680 v4 @ 2.40GHz
|Memory              |0.66GB/2.02GB
|                    |
|V executable        |/home/admin/v/v
|V last modified time|2025-05-18 12:25:29
|                    |
|V home dir          |OK, value: /home/admin/v
|VMODULES            |OK, value: .vmodules
|VTMP                |OK, value: /tmp/v_0
|Current working dir |OK, value: /home/admin/playground
|                    |
|Git version         |git version 2.39.5
|V git status        |N/A
|.git/config present |true
|                    |
|cc version          |cc (Debian 12.2.0-14) 12.2.0
|gcc version         |gcc (Debian 12.2.0-14) 12.2.0
|clang version       |Debian clang version 14.0.6
|tcc version         |tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
|tcc git status      |Error: fatal: detected dubious ownership in repository at '/home/admin/v/thirdparty/tcc'
To add an exception for this directory, call:

	git config --global --add safe.directory /home/admin/v/thirdparty/tcc
 Error: fatal: detected dubious ownership in repository at '/home/admin/v/thirdparty/tcc'
To add an exception for this directory, call:

	git config --global --add safe.directory /home/admin/v/thirdparty/tcc

|emcc version        |N/A
|glibc version       |ldd (Debian GLIBC 2.36-9+deb12u8) 2.36

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