cgen: fix option with multi return assign (#18174)

vlib/v/gen/c/assign.v

@@ -747,7 +747,10 @@ fn (mut g Gen) gen_multi_return_assign(node &ast.AssignStmt, return_type ast.Typ
 				g.write(' = ${tmp_var};')
 			} else {
 				g.write('_option_ok(&(${base_typ}[]) { ${tmp_var} }, (${option_name}*)(&')
+				tmp_left_is_opt := g.left_is_opt
+				g.left_is_opt = true
 				g.expr(lx)
+				g.left_is_opt = tmp_left_is_opt
 				g.writeln('), sizeof(${base_typ}));')
 			}
 		} else {

vlib/v/tests/option_multi_return_assign_test.v

@@ -0,0 +1,11 @@
+fn t() (string, string) {
+	return 'foo', 'bar'
+}
+
+fn test_main() {
+	mut a := ?string(none)
+	mut b := ''
+	a, b = t()
+	assert a? == 'foo'
+	assert b == 'bar'
+}