Make all Literal default-constructible; fixes #536 (#539)

Author: liuzicheng1987

include/rfl/Literal.hpp

@@ -45,10 +45,6 @@ class Literal {
 
   Literal(const std::string& _str) : value_(find_value(_str).value()) {}
 
-  /// A single-field literal is special because it
-  /// can also have a default constructor.
-  template <ValueType num_fields = num_fields_,
-            typename = std::enable_if_t<num_fields <= 1>>
   Literal() : value_(0) {}
 
   ~Literal() = default;

tests/json/test_duration_default.cpp

@@ -0,0 +1,19 @@
+#include <chrono>
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+
+#include "rfl/DefaultIfMissing.hpp"
+#include "write_and_read.hpp"
+
+namespace test_duration_default {
+
+struct TestStruct {
+  std::chrono::seconds duration;
+};
+
+TEST(json, test_duration_default) {
+  const auto test = TestStruct{.duration = std::chrono::seconds(10)};
+  write_and_read<rfl::DefaultIfMissing>(
+      test, R"({"duration":{"count":10,"unit":"seconds"}})");
+}
+}  // namespace test_duration_default