checker: struct defined on function taking vararg as parameter can be misdetected an non-vararg

[dy-tea]

Describe the bug

Reported by @koplenov.

Made a smaller repro of his issue below.

Reproduction Steps

struct As {
}

type Aa = string | int

fn (s As) b(aa ...Aa) int {
	return 0
}

fn (s As) a(s2 &As, aa ...Aa) int {
	a := s2.b(aa)
	return a
}

fn main() {
	s := As{}
	a := []Aa{}
	println(s.a(&s, ...a))
}

Expected Behavior

Should print 0.

Current Behavior

❯ v run test.v
test.v:11:10: error: to pass `aa` (Aa) to `b` (which accepts type `...Aa`), use `...aa`
    9 |
   10 | fn (s As) a(s2 &As, aa ...Aa) int {
   11 |     a := s2.b(aa)
      |             ~~~~~
   12 |     return a
   13 | }

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.12 8342968

Environment details (OS name and version, etc.)

V full version	V 0.4.12 8342968
OS	linux, "CachyOS"
Processor	12 cpus, 64bit, little endian, 13th Gen Intel(R) Core(TM) i5-1335U
Memory	1.92GB/15.31GB
V executable	/home/dylan/Repos/v/v
V last modified time	2025-10-14 15:06:26
V home dir	OK, value: /home/dylan/Repos/v
VMODULES	OK, value: /home/dylan/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/dylan/Repos/v
Git version	git version 2.51.0
V git status	weekly.2025.42-6-g6a0b15bd
.git/config present	true
cc version	cc (GCC) 15.2.1 20250813
gcc version	gcc (GCC) 15.2.1 20250813
clang version	clang version 20.1.8
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (GNU libc) 2.42

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