Operator `+=` is not implemented for generic type alias `type V2 = vec.Vec2[f32]`, even though `vlib/math/vec/vec2.v:75` implements it for the parent type `Vec2[T]`

[spytheman]

V version: V 0.4.9 21874f9, press to see full `v doctor` output

V full version	V 0.4.9 c69b125.21874f9
OS	linux, Ubuntu 20.04.6 LTS
Processor	4 cpus, 64bit, little endian, Intel(R) Core(TM) i3-3225 CPU @ 3.30GHz
Memory	0.42GB/15.05GB
V executable	/home/delian/v/v
V last modified time	2025-03-17 07:08:53
V home dir	OK, value: /home/delian/v
VMODULES	OK, value: /home/delian/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/delian/v
env VFLAGS	"-no-parallel"
Git version	git version 2.48.1
V git status	weekly.2025.11-48-g5978cbcf-dirty
.git/config present	true
cc version	cc (Ubuntu 10.5.0-1ubuntu1~20.04) 10.5.0
gcc version	gcc (Ubuntu 10.5.0-1ubuntu1~20.04) 10.5.0
clang version	clang version 10.0.0-4ubuntu1
tcc version	tcc version 0.9.28rc 2024-07-31 HEAD@1cee0908 (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 0134e9b9
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.31-0ubuntu9.16) 2.31

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

import math.vec

type V2 = vec.Vec2[f32]

///// When this is uncommented, the program compiles and runs properly
//fn (a V2) + (b V2) V2 { return a.add(b) }

fn main() {
	a := V2{1,1}
	b := V2{10,20}
	mut c := a + b
	dump(c)
	c += a
	dump(c)
	assert c.x == 12
	assert c.y == 22
}
	

What did you see?

b.v:13:4: error: undefined operation `V2` + `V2`
   11 |     mut c := a + b
   12 |     dump(c)
   13 |     c += a
      |       ~~
   14 |     dump(c)
   15 |     assert c.x == 12

What did you expect to see?

A compiled program, since math.vec does define the + operator in vlib/math/vec/vec2.v:75.
Note also that c := a + b works, it is only c += a that does not get inferred correctly and produces an error.

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[huly-for-github]

Connected to Huly®: V_0.6-22360

[Delta456]

I know what causes the issue. I will get to this.