support int const and friends

Author: ConorWilliams

include/libfork/ev.hpp

@@ -24,7 +24,7 @@ namespace lf {
  * @tparam T The type to test.
  */
 template <typename T>
-concept trivial_return = std::default_initializable<T> && std::is_trivially_destructible_v<T>;
+concept trivial_return = std::constructible_from<T> && std::is_trivially_destructible_v<T>;
 
 /**
  * @brief A wrapper for return values from libfork's coroutines.
@@ -61,6 +61,18 @@ class ev : detail::immovable<ev<T>> {
 template <trivial_return T>
 class ev<T> : detail::immovable<ev<T>> {
  public:
+  constexpr ev() = default;
+
+  /*
+   * @brief Default construct
+   *
+   * This is required to subsume the trivial constuctor for
+   * non default-initializable types like `int const`.
+   */
+  constexpr ev() noexcept
+    requires (!requires { ::new T; })
+      : m_value() {}
+
   template <typename Self>
   [[nodiscard]] constexpr auto operator*(this Self &&self) noexcept -> auto && {
     return std::forward<Self>(self).m_value;

test/src/test_ev.cpp

@@ -9,13 +9,8 @@
 
 namespace {
 
-template <typename T, typename Ref>
-concept deref_like = requires (T val, Ref ref) {
-  { *val } -> std::same_as<decltype((ref))>;
-  { *std::move(val) } -> std::same_as<decltype(std::move(ref))>;
-};
-
 struct non_trivial {
+  int x;
   constexpr ~non_trivial() {}
 };
 
@@ -25,18 +20,24 @@ consteval auto const_test() -> bool {
 
   {
     lf::ev<non_trivial const> x;
-    x.emplace();
+    x.emplace(5);
   }
 
   {
-    lf::ev<int> x;
+    lf::ev<int const> x;
     int const *p = x.get();
     std::construct_at(p, 3);
   }
 
   return true;
 }
 
+template <typename T, typename Ref>
+concept deref_like = requires (T val, Ref ref) {
+  { *val } -> std::same_as<decltype((ref))>;
+  { *std::move(val) } -> std::same_as<decltype(std::move(ref))>;
+};
+
 } // namespace
 
 TEST_CASE("Ev constexpr tests", "[ev]") { REQUIRE(const_test()); }