Enable writing to force_opt lens with non-opt value.

Author: Siapran

lager/lenses/optional.hpp

@@ -70,6 +70,14 @@ struct add_opt
     using type = std::optional<std::decay_t<T>>;
 };
 
+template <typename T>
+struct is_optional : std::false_type
+{};
+
+template <typename T>
+struct is_optional<std::optional<T>> : std::true_type
+{};
+
 } // namespace detail
 
 //! @defgroup lenses
@@ -110,7 +118,7 @@ auto value_or(T&& t)
 {
     return zug::comp([t = std::forward<T>(t)](auto&& f) {
         return [&, f = LAGER_FWD(f)](auto&& whole) {
-            return f(LAGER_FWD(whole).value_or(std::move(t)))(
+            return f(LAGER_FWD(whole).value_or(t))(
                 [&](auto&& x) { return LAGER_FWD(x); });
         };
     });
@@ -141,8 +149,13 @@ ZUG_INLINE_CONSTEXPR auto or_default = value_or();
 ZUG_INLINE_CONSTEXPR auto force_opt = zug::comp([](auto&& f) {
     return [f = LAGER_FWD(f)](auto&& p) {
         using opt_t = std::optional<std::decay_t<decltype(p)>>;
-        return f(opt_t{LAGER_FWD(p)})(
-            [&](auto&& x) { return LAGER_FWD(x).value_or(LAGER_FWD(p)); });
+        auto opt    = opt_t{LAGER_FWD(p)};
+        return f(std::move(opt))([&](auto&& x) -> decltype(auto) {
+            if constexpr (detail::is_optional<std::decay_t<decltype(x)>>::value)
+                return LAGER_FWD(x).value_or(*std::move(opt));
+            else
+                return LAGER_FWD(x);
+        });
     };
 });
 

test/lenses.cpp

@@ -21,8 +21,8 @@
 #include <lager/lenses/at_or.hpp>
 #include <lager/lenses/attr.hpp>
 #include <lager/lenses/optional.hpp>
-#include <lager/lenses/variant.hpp>
 #include <lager/lenses/tuple.hpp>
+#include <lager/lenses/variant.hpp>
 
 #include <array>
 
@@ -423,6 +423,21 @@ TEST_CASE("lenses, bind_opt")
     }
 }
 
+
+TEST_CASE("lenses::force_opt with bind_opt", "[lenses][force_opt][bind_opt]")
+{
+    auto opt_member_lens  = attr(&yearday::day) | force_opt;
+    auto bound_opt_member = bind_opt(opt_member_lens);
+    
+    CHECK(view(bound_opt_member, std::optional{yearday{1, 1}}) ==
+          std::optional{1});
+    CHECK(view(bound_opt_member, std::optional<yearday>{}) == std::nullopt);
+    CHECK(set(bound_opt_member, std::optional{yearday{1, 1}}, 1)->day == 1);
+    CHECK(set(bound_opt_member, std::optional{yearday{1, 1}}, std::optional{1})
+              ->day == 1);
+    CHECK(!set(bound_opt_member, std::optional<yearday>{}, std::nullopt));
+}
+
 TEST_CASE("lenses::zip pair", "[lenses][zip][pair]")
 {
     struct foo
@@ -433,9 +448,8 @@ TEST_CASE("lenses::zip pair", "[lenses][zip][pair]")
     std::pair<foo, int> baz{{42}, 256};
     auto zipped = zip(attr(&foo::value), lager::identity);
 
-    baz = over(zipped, baz, [](auto x) {
-        return std::pair{x.second, x.first};
-    });
+    baz =
+        over(zipped, baz, [](auto x) { return std::pair{x.second, x.first}; });
 
     CHECK(baz.first.value == 256);
     CHECK(baz.second == 42);