cgen: `match` with `!`-propagating arms used as an if-expression value emits `_t4 = ;` (expected expression)

[medvednikov]

V version: V 0.5.2 (master)
OS: Linux (reported via bugs.vlang.io)

Description

A match whose arms use ! error-propagation (f(value)!), when the match is used as the value of an if-expression (here an if value := opt { … } else { … } option-guard assigned to a variable), emits invalid C: the result temp is assigned an empty expression — _t4 = ; — giving expected expression. Replacing the !-propagating arms with plain expressions compiles fine, so the trigger is !-propagation inside a match-as-if-expr-value.

Reproduction

module main

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{})!)
}

Actual (generated C fails to compile)

error: expected expression
    _t4 = ;

The result temp for the if/match expression is never assigned the propagated value.

Expected

Compiles; the match arms' !-propagated results are assigned to the if-expression's result temp.

Note

The identical code with non-! arms (First { 1 } Second { 2 }, and a non-! return type) compiles and runs correctly.

Reported via bugs.vlang.io

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.