jsgen: alias types are not properly resolved in sum types

[gchumillas]

Describe the bug

Alias types are not properly resolved when sum types are involved.

Reproduction Steps

Write a simple app:

type Type0 = string
type Type1 = Type0 | int

struct Foo {
	field Type1
}

fn main() {
	foo := Foo{}

	if foo.field is Type0 {
		assert foo.field == Type0('')
	} else {
		assert false
	}
}

Compile to JavaScript and execute it:

v -b js main.v
node main.js

Output:

FAIL: fn main.main(): assert false

Expected Behavior

It should NOT fail.

Current Behavior

It fails.

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.10 2ca9209

Environment details (OS name and version, etc.)

V full version	V 0.4.10 dead5e6.2ca9209
OS	macos, macOS, 15.4.1, 24E263
Processor	8 cpus, 64bit, little endian, Apple M1 Pro
Memory	0.15GB/16GB
V executable	/Users/gonzalo/Projects/Personal/v/nv/v
V last modified time	2025-05-14 14:46:58
V home dir	OK, value: /Users/gonzalo/Projects/Personal/v/nv
VMODULES	OK, value: /Users/gonzalo/.vmodules
VTMP	OK, value: /tmp/v_501
Current working dir	OK, value: /Users/gonzalo/Projects/Personal/v/nv
Git version	git version 2.39.1
V git status	weekly.2025.19-33-g2ca92090
.git/config present	true
cc version	Apple clang version 17.0.0 (clang-1700.0.13.3)
gcc version	Apple clang version 17.0.0 (clang-1700.0.13.3)
clang version	Apple clang version 17.0.0 (clang-1700.0.13.3)
tcc version	tcc version 0.9.28rc 2024-02-05 HEAD@105d70f7 (AArch64 Darwin)
tcc git status	thirdparty-macos-arm64 e447816
emcc version	N/A
glibc version	N/A

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.

[huly-for-github]

Connected to Huly®: V_0.6-22855