C error on string concat with comptime and generics

[islonely]

V doctor:

V full version: V 0.4.9 4225a34.7cf77fb
OS: macos, macOS, 15.1.1, 24B91
Processor: 8 cpus, 64bit, little endian, Apple M3

getwd: /Users/adamoates/.vmodules/islonely/hex
vexe: /Users/adamoates/v/v
vexe mtime: 2025-01-02 04:35:14

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

Git version: git version 2.39.5 (Apple Git-154)
Git vroot status: weekly.2024.53-10-g7cf77fbd
.git/config present: true

CC version: Apple clang version 16.0.0 (clang-1600.0.26.4)
emcc version: N/A
thirdparty/tcc status: thirdparty-macos-arm64 713692d4

What did you do?
./v -g -o vdbg cmd/v && ./vdbg /Users/adamoates/Documents/test.v && /Users/adamoates/Documents/test

fn main() {
	println(enc(Test{}))
}
enum Flag {
	usa_old_glory
	all_other_bad_excuses_for_a_flag
}

struct Test {
	is_foo bool
	name [5]u8
}

fn enc[T](item T) string {
	$if T is $int {
		len := match typeof(item).name {
			'i8', 'u8' { u8(2) }
			'i16', 'u16' { 4 }
			'int', 'u32', 'i32' { 8 }
			'i64', 'u64' { 16 }
			else { return '' }
		}
		return u64_to_hex(item, len)
	} $else $if T is $array {
		mut hex := ''
		for val in item {
			hex += enc(val)
		}
		return hex
	} $else $if T is $struct {
		mut hex := ''
		$for field in T.fields {
			hex += enc(item.$(field.name)) + '_'
		}
		return hex
	} $else {
		if typeof(item).name == 'bool' {
			return enc(int(item))
		}
		$if debug {
			println('cannot encode ${T}(s)')
		}
		return ''
	}
}

@[direct_array_access; inline]
fn u64_to_hex(nn u64, len u8) string {
	mut n := nn
	mut buf := [17]u8{}
	buf[len] = 0
	mut i := 0
	for i = len - 1; i >= 0; i-- {
		d := u8(n & 0xF)
		buf[i] = if d < 10 { d + `0` } else { d + 87 }
		n = n >> 4
	}
	return unsafe { tos(memdup(&buf[0], len + 1), len) }
}

What did you see instead?

================== C compilation error (from cc): ==============
cc: /tmp/v_501/test.01JGJX16A7PGJ1TBVGE9ANTRWG.tmp.c:5852:42: error: call to undeclared function 'main__enc_T_Array_fixed_u8_5'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
cc:  5852 |                                 hex = string__plus(hex, string__plus(main__enc_T_Array_fixed_u8_5(item.name), _SLIT("_")));
cc:       |                                                                      ^
cc: /tmp/v_501/test.01JGJX16A7PGJ1TBVGE9ANTRWG.tmp.c:5852:42: error: passing 'int' to parameter of incompatible type 'string' (aka 'struct string')
cc:  5852 |                                 hex = string__plus(hex, string__plus(main__enc_T_Array_fixed_u8_5(item.name), _SLIT("_")));
cc:       |                                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc: /tmp/v_501/test.01JGJX16A7PGJ1TBVGE9ANTRWG.tmp.c:4923:44: note: passing argument to parameter 's' here
cc:  4923 | VV_LOCAL_SYMBOL string string__plus(string s, string a) {
cc:       |                                            ^
cc: 2 errors generated.
================================================================
(You can pass `-cg`, or `-show-c-output` as well, to print all the C error messages).
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 .

What did you expect to see?

Expected code to compile and output a line of zeros.

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_{Huly®: V_0.6-21772}