fix optional struct field assignment

vlib/compiler/parser.v

@@ -1441,7 +1441,7 @@ fn ($v.name mut $v.typ) ${p.cur_fn.name}(...) {
 		expr := p.cgen.cur_line[pos..]
 		left := p.cgen.cur_line[..pos]
 		typ := expr_type.replace('Option_', '')
-		p.cgen.resetln(left + 'opt_ok($expr, sizeof($typ))')
+		p.cgen.resetln(left + 'opt_ok(($typ[]){ $expr }, sizeof($typ))')
 	}
 	else if expr_type.starts_with('Option_') &&
 		p.assigned_type == expr_type['Option_'.len..] && p.tok == .key_orelse

vlib/compiler/tests/option_test.v

@@ -112,3 +112,15 @@ fn test_field_or() {
 	}
 	assert mytitle == 'default'
 }
+
+struct Thing {
+mut:
+	opt ?int
+}
+
+fn test_opt_field() {
+	mut t := Thing{}
+	t.opt = 5
+	val := t.opt or { return }
+	assert val == 5
+}