cgen: C backend emits an empty assignment for Result propagation in a sum-type match nested in an if expression

[skandhas]

Describe the bug

When a sum-type match block is nested inside an if expression, using the ! operator to propagate a Result type inside the match branches causes a C compilation error.

The V compiler successfully passes semantic checks. However, it looks like the generated C code emits an incomplete/empty assignment statement (e.g., _t4 = ;) for the temporary variables intended to capture the match branch results, resulting in a syntax error (error: expected expression before ';' token) from the underlying C compiler.

Reproduction Steps

struct First {}
struct Second {}

type Node = First | Second

fn lower_first(_ First) !int {
	return 1
}

fn lower_second(_ Second) !int {
	return 2
}

fn select_value(node ?Node) !int {
	result := if value := node {
		match value {
			First {
				lower_first(value)!
			}
			Second {
				lower_second(value)!
			}
		}
	} else {
		0
	}

	return result
}

fn main() {
	println(select_value(First{})!)
}

Expected Behavior

The code should compile successfully without any C compiler errors and output 1.

Current Behavior

warning: tcc compilation failed, falling back to gcc
================== C compilation error (from gcc): ==============
cc: C:\\Users\\skand\\AppData\\Local\\Temp\\v_0\\ISC65B~1.C: In function 'main__select_value':
cc: C:\\Users\\skand\\AppData\\Local\\Temp\\v_0\\ISC65B~1.C:4670:31: error: expected expression before ';' token
cc:  4670 |                         _t4 = ;
cc:       |                               ^
cc: C:\\Users\\skand\\AppData\\Local\\Temp\\v_0\\ISC65B~1.C:4681:31: error: expected expression before ';' token
cc:  4681 |                         _t4 = ;
cc:       |                               ^
cc: At top level:
cc: cc1.exe: note: unrecognized command-line option '-Wno-incompatible-function-pointer-types' may have been intended to silence earlier diagnostics
...
cc: cc1.exe: note: unrecognized command-line option '-Wno-incompatible-function-pointer-types' may have been intended to silence earlier diagnostics
(note: the original output was 26 lines long; it was truncated to its first 9 lines + the last line)
=================================================================
Try passing `-g` when compiling, to see a .v file:line information, that correlates more with the C error.
(Alternatively, pass `-show-c-output`, to print the full C error message).
================== C compiler bug report ==============
Sent C compiler bug report to https://bugs.vlang.io/bug-report.
Report id: bdc2946f-2b8d-41f2-8654-5a0138a1d980
Delete this report from the server with:
  curl -X DELETE "https://bugs.vlang.io/bug-report/bdc2946f-2b8d-41f2-8654-5a0138a1d980?token=75b3fc86-3610-4626-be1b-d5fba7cf1411"
V V 0.5.2 09062b4577f57aa3ca57adb528d751824d8f51a8.be51760, Windows/amd64, cc: gcc, build options: gc:boehm_full_opt skip_unused -d gcboehm -d gcboehm_full -d gcboehm_opt -d windows
Generated C lines sent from C:\Users\skand\AppData\Local\Temp\v_0\issue_2_sum_type_match_in_if.01KYRWQN0SKTBJ47VV4K8CYNPD.tmp.c:0:
  (no source lines available)
Corresponding V lines sent from C:\\Users\\skand\\AppData\\Local\\Temp\\v_0\\ISC65B~1.C:4670:
    4665 |                              _t6.is_error = true;
    4666 |                              _t6.err = _t5.err;
    4667 |                              return _t6;
    4668 |                      }
    4669 |
>   4670 |                      _t4 = ;
    4671 |              }
    4672 |              else if (value._typ == 150) {
    4673 | _result_int _t7 = main__lower_second((*value._main__Second));
    4674 |                      if (_t7.is_error) {
    4675 |                              _result_int _t8 = {0};
=======================================================
builder error:
==================
C error found while compiling generated C code.
This can be caused by invalid C interop code, C compiler flags, or a V compiler bug.
If your code is pure V and this still happens, 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 .

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.5.2 be51760

Environment details (OS name and version, etc.)

|V full version      |V 0.5.2 09062b4577f57aa3ca57adb528d751824d8f51a8.be51760
|:-------------------|:-------------------
|OS                  |windows, Microsoft Windows 11 Home 26200 64-bit
|Processor           |16 cpus, 64bit, little endian, 12th Gen Intel(R) Core(TM) i5-1240P
|Memory              |17.57GB/31.58GB
|                    |
|V executable        |D:\sdks\v\v.exe
|V last modified time|2026-07-30 06:37:26
|                    |
|V home dir          |OK, value: D:\sdks\v
|VMODULES            |OK, value: C:\Users\skand\.vmodules
|VTMP                |OK, value: C:\Users\skand\AppData\Local\Temp\v_0
|Current working dir |OK, value: D:\workd\vtools\bug\vbug-repro
|                    |
|Git version         |git version 2.51.1
|V git status        |be517609
|.git/config present |true
|                    |
|cc version          |cc (Rev3, Built by MSYS2 project) 16.1.0
|gcc version         |gcc (Rev3, Built by MSYS2 project) 16.1.0
|clang version       |clang version 22.1.7 (https://github.com/msys2/MINGW-packages b8f306ad57e0c8f3f15aed0dc809d7e3e8e2eee9)
|msvc version        |N/A
|tcc version         |tcc version 0.9.28rc 2026-07-14_HEAD@d9d02c56* (x86_64 Windows)
|tcc git status      |thirdparty-windows-amd64 f7c7199b-dirty
|emcc version        |N/A
|glibc version       |ldd (cygwin) 3.6.9

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