some bug with comptime?

[dy-tea]

Describe the bug

Below is my (incorrect) implementation of offsetof.

Code: https://play.vlang.io/p/321ea89b0c

pub fn offsetof[T](member_name string) int {
	$if T is $struct {
		$for field in T.fields {
			if field.name == member_name {
				tmp := T{}
				access := int(tmp.$(field.name)?)
				return access
			}
		}
	} $else {
		$compile_error('offsetof only accepts struct types')
	}
	return 0
}

struct Test {
	a int
	b string
	c int
}

fn main() {
	println(offsetof[Test]('c'))
}

Reproduction Steps

See included code.

Expected Behavior

Not sure honestly.

Current Behavior

Output:

/box/code.v:6: error: struct or union expected
builder error: 
==================
C error found. It should never happen, when compiling pure V code.
This is a V compiler bug, please report it using `v bug file.v`,
or goto https://github.com/vlang/v/issues/new/choose .
You can also use #help on Discord: https://discord.gg/vlang .
Exited with error status 1

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.12 abe97b6

Environment details (OS name and version, etc.)

|V full version      |V 0.4.12 abe97b636b16b37eb996bfd45c8b6a28d157b2cf
|:-------------------|:-------------------
|OS                  |linux, Debian GNU/Linux 12 (bookworm) (VM)
|Processor           |2 cpus, 64bit, little endian, Intel(R) Xeon(R) CPU E5-2680 v4 @ 2.40GHz
|Memory              |0.57GB/2.02GB
|                    |
|V executable        |/home/admin/v/v
|V last modified time|2025-12-31 17:25:31
|                    |
|V home dir          |OK, value: /home/admin/v
|VMODULES            |OK, value: .vmodules
|VTMP                |OK, value: /tmp/v_0
|Current working dir |OK, value: /home/admin/playground
|                    |
|Git version         |git version 2.39.5
|V git status        |N/A
|.git/config present |true
|                    |
|cc version          |cc (Debian 12.2.0-14+deb12u1) 12.2.0
|gcc version         |gcc (Debian 12.2.0-14+deb12u1) 12.2.0
|clang version       |Debian clang version 14.0.6
|tcc version         |tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
|tcc git status      |Error: fatal: detected dubious ownership in repository at '/home/admin/v/thirdparty/tcc'
To add an exception for this directory, call:

	git config --global --add safe.directory /home/admin/v/thirdparty/tcc
 Error: fatal: detected dubious ownership in repository at '/home/admin/v/thirdparty/tcc'
To add an exception for this directory, call:

	git config --global --add safe.directory /home/admin/v/thirdparty/tcc

|emcc version        |N/A
|glibc version       |ldd (Debian GLIBC 2.36-9+deb12u10) 2.36

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[dy-tea]

Closer to what I was trying to accomplish:

pub fn offsetof[T](member_name string) int {
	$if T is $struct {
		$for field in T.fields {
			if field.name == member_name {
				tmp := T{}
				access := &(tmp.$(field.name)?)
				return unsafe {int(access) - int(&tmp)}
			}
		}
	} $else {
		$compile_error('offsetof only accepts struct types')
	}
	return 0
}

[dy-tea]

Here is my working offsetof if anyone is interested:

pub fn offsetof[T](member_name string) int {
	tmp := &T{}
	$if T is $struct {
		$for field in T.fields {
			if field.name == member_name {
				access := int(voidptr(&tmp.$(field.name)))
				return access - int(tmp)
			}
		}
	} $else {
		$compile_error('offsetof only accepts struct types')
	}
	return 0
}

[felipensp]

The problem that you are using tmp.$(field.name)? for option and non-option field.

[dy-tea]

Yes it should be a checker bug.

[felipensp]

			if field.name == member_name {
				tmp := T{}
				$if field.typ is $option {
					access := int(tmp.$(field.name)?)
					return access
				} $else $if field.typ is $string {
					return tmp.$(field.name).int()
				} $else {
					return int(tmp.$(field.name))
				}				
			}