cgen: fix multi return variable ignore in if guard (#17853)

vlib/v/gen/c/if.v

@@ -285,7 +285,7 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 					g.writeln(', !${var_name}.is_error) {')
 				}
 			}
-			if short_opt || branch.cond.vars[0].name != '_' {
+			if short_opt || branch.cond.vars.len > 1 || branch.cond.vars[0].name != '_' {
 				base_type := g.base_type(branch.cond.expr_type)
 				if short_opt {
 					cond_var_name := if branch.cond.vars[0].name == '_' {
@@ -316,6 +316,9 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 						if sym.info is ast.MultiReturn {
 							if sym.info.types.len == branch.cond.vars.len {
 								for vi, var in branch.cond.vars {
+									if var.name == '_' {
+										continue
+									}
 									var_typ := g.typ(sym.info.types[vi])
 									left_var_name := c_name(var.name)
 									if is_auto_heap {

vlib/v/gen/c/testdata/multi_return_ignored_if_guard.out

@@ -0,0 +1,32 @@
+1
+2
+1
+2
+1
+2
+1
+2
+1
+2
+3
+1
+2
+1
+3
+1
+2
+3
+2
+3
+1
+2
+3
+1
+2
+1
+3
+1
+2
+3
+2
+3

vlib/v/gen/c/testdata/multi_return_ignored_if_guard.vv

@@ -0,0 +1,98 @@
+fn f1() !(int, int) {
+	return 1, 2
+}
+
+fn f2() ?(int, int) {
+	return 1, 2
+}
+
+fn f3() !(int, int, int) {
+	return 1, 2, 3
+}
+
+fn f4() ?(int, int, int) {
+	return 1, 2, 3
+}
+
+fn main() {
+	if l, r := f1() {
+		println(l)
+		println(r)
+	}
+	if l, _ := f1() {
+		println(l)
+	}
+	if _, r := f1() {
+		println(r)
+	}
+	if _, _ := f1() {
+	}
+	if l, r := f2() {
+		println(l)
+		println(r)
+	}
+	if l, _ := f2() {
+		println(l)
+	}
+	if _, r := f2() {
+		println(r)
+	}
+	if _, _ := f2() {
+	}
+	if x, y, z := f3() {
+		println(x)
+		println(y)
+		println(z)
+	}
+	if x, y, _ := f3() {
+		println(x)
+		println(y)
+	}
+	if x, _, z := f3() {
+		println(x)
+		println(z)
+	}
+	if x, _, _ := f3() {
+		println(x)
+	}
+	if _, y, z := f3() {
+		println(y)
+		println(z)
+	}
+	if _, y, _ := f3() {
+		println(y)
+	}
+	if _, _, z := f3() {
+		println(z)
+	}
+	if _, _, _ := f3() {
+	}
+	if x, y, z := f4() {
+		println(x)
+		println(y)
+		println(z)
+	}
+	if x, y, _ := f4() {
+		println(x)
+		println(y)
+	}
+	if x, _, z := f4() {
+		println(x)
+		println(z)
+	}
+	if x, _, _ := f4() {
+		println(x)
+	}
+	if _, y, z := f4() {
+		println(y)
+		println(z)
+	}
+	if _, y, _ := f4() {
+		println(y)
+	}
+	if _, _, z := f4() {
+		println(z)
+	}
+	if _, _, _ := f4() {
+	}
+}