In the out-argument overload of rule_parser::call(), always parse into a local

attr_type attribute ('attr'), rather than the one supplied ('retval').  Then,
take the parsed attribute and either combine it with retval (if they are both
containers), or assign attr to retval (if attr_type is not none).

Reproducer by Andreas Buhr.

Fixes #294.

test/github_issues.cpp

@@ -512,6 +512,45 @@ void github_pr_290()
     BOOST_TEST(*result == "foo");
 }
 
+namespace github_issue_294_ {
+    namespace bp = boost::parser;
+    struct Foo
+    {};
+    constexpr bp::rule<struct foo_parser_tag, std::shared_ptr<Foo>> foo_parser =
+        "foo_parser";
+    constexpr auto foo_parser_action = [](auto & ctx) {
+        std::shared_ptr<Foo> & val = _val(ctx);
+        val = std::shared_ptr<Foo>(new Foo{});
+    };
+    constexpr auto foo_parser_def = bp::eps[foo_parser_action];
+    struct Bar
+    {
+        std::shared_ptr<Foo> foo;
+    };
+    constexpr bp::rule<struct bar_parser_tag, std::shared_ptr<Bar>> bar_parser =
+        "bar_parser";
+    constexpr auto bar_parser_action = [](auto & ctx) {
+        std::shared_ptr<Bar> & val = _val(ctx);
+        val = std::shared_ptr<Bar>(new Bar{});
+        std::optional<std::shared_ptr<Foo>> & attr = _attr(ctx);
+        if (attr) {
+            val->foo = attr.value();
+        }
+    };
+    constexpr auto bar_parser_def =
+        (bp::lit("(") > -foo_parser > bp::lit(")"))[bar_parser_action];
+
+    BOOST_PARSER_DEFINE_RULES(bar_parser, foo_parser);
+}
+
+void github_issue_294()
+{
+    namespace bp = boost::parser;
+    using namespace github_issue_294_;
+
+    bp::parse("()", bar_parser, bp::blank);
+}
+
 namespace github_pr_297_ {
     namespace bp = boost::parser;
     constexpr auto bar_required_f = [](auto& ctx) -> bool {
@@ -558,6 +597,7 @@ void github_pr_297()
     }
 }
 
+
 int main()
 {
     github_issue_36();
@@ -573,6 +613,7 @@ int main()
     github_issue_279();
     github_issue_285();
     github_pr_290();
+    github_issue_294();
     github_pr_297();
     return boost::report_errors();
 }