checker, generics: X is T does not work

[Le0Developer]

V doctor:

V full version: V 0.4.9 5b44b67
OS: macos, macOS, 15.2, 24C101
Processor: 8 cpus, 64bit, little endian, Apple M2

getwd: /Users/leodev/p/v/cluedo
vexe: /Users/leodev/p/v/v/v
vexe mtime: 2024-12-27 12:16:03

vroot: OK, value: /Users/leodev/p/v/v
VMODULES: OK, value: /Users/leodev/.vmodules
VTMP: OK, value: /tmp/v_501

Git version: git version 2.39.5 (Apple Git-154)
Git vroot status: weekly.2024.52-6-g5b44b672
.git/config present: true

CC version: Apple clang version 16.0.0 (clang-1600.0.26.6)
emcc version: N/A
thirdparty/tcc status: thirdparty-macos-arm64 713692d4

What did you do?
./v -g -o vdbg cmd/v && ./vdbg repro.v

type Sumtype = string | int

fn generic_fn[T]() ?T {
	a := Sumtype('123')
	if a is T {
		return a
	}

	b := Sumtype(123)
	if b is T {
		return b
	}

	return none
}

fn main() {
	println(generic_fn[string]())
	println(generic_fn[int]())
}

What did you expect to see?

Successful compilation

What did you see instead?

repro.v:6:10: error: `Sumtype` has no variant `T`
    4 | fn generic_fn[T]() ?T {
    5 |     a := Sumtype('123')
    6 |     if a is T {
      |             ^
    7 |         return a
    8 |     }
repro.v:11:10: error: `Sumtype` has no variant `T`
    9 | 
   10 |     b := Sumtype(123)
   11 |     if b is T {
      |             ^
   12 |         return b
   13 |     }

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_{Huly®: V_0.6-21714}