Invalid generic return type syntax is not caught and causes side effects

[erickilisboars]

Describe the bug

The compiler fails to catch an incorrect syntax for generic return types. Using [T] instead of T as a return type results in a cgen error and incorrect code rewriting when using v fmt.

Reproduction Steps

Create a file named main.v and then run v run main.v:

module main

fn foo[T](v T) [T] { // <- wrong return type syntax
    return v
}

fn main() {
    println(foo[string]('Hello World'))
}

Expected Behavior

A syntax error should be emitted.

Current Behavior

1. Compilation error: Running v run main.v produces a low-level C generation error instead of a syntax error:

   cgen error: could not generate string method `builtin__void_str` for type `void`
   

2. v fmt corruption: Running v fmt -w main.v transforms the function signature into invalid code:

   fn foo[T](v T) [T]void {
       return v
   }

   The formatter appends void to the brackets.

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.5.0

Environment details (OS name and version, etc.)

V full version	V 0.5.0 50a7d53.11bf3dd
OS	linux, Ubuntu 24.04.3 LTS (WSL 2)
Processor	6 cpus, 64bit, little endian, AMD Ryzen 5 3500 6-Core Processor
Memory	4.51GB/7.72GB
V executable	/v/v
V last modified time	2026-02-05 09:24:30
V home dir	OK, value: /v
VMODULES	OK, value: /root/.vmodules
VTMP	OK, value: /tmp/v_0
Current working dir	OK, value: /workspace
Git version	git version 2.43.0
V git status	11bf3dd
.git/config present	true
cc version	cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
gcc version	gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
clang version	Ubuntu clang version 18.1.3 (1ubuntu1)
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 (Ubuntu GLIBC 2.39-0ubuntu8.6) 2.39

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