Fix construction from empty optional<U>

Fixes bemanproject#49 for the constructor case. Adds failing tests, and fixes them.

Author: steve-downey

include/Beman/Optional26/optional.hpp

@@ -1044,7 +1044,13 @@ class optional<T&> {
     }
 
     template <class U>
-    constexpr explicit(!std::is_convertible_v<U, T>) optional(const optional<U>& rhs) noexcept : optional(*rhs) {}
+    constexpr explicit(!std::is_convertible_v<U, T>) optional(const optional<U>& rhs) noexcept {
+        static_assert(std::is_constructible_v<std::add_lvalue_reference_t<T>, U>, "Must be able to bind U to T&");
+        if (rhs.has_value())
+            value_ = std::to_address(rhs);
+        else
+            value_ = nullptr;
+    }
 
     //  \rSec3[optional.dtor]{Destructor}
 

src/Beman/Optional26/tests/optional_ref.t.cpp

@@ -620,3 +620,39 @@ TEST(OptionalRefTest, AssignFromOptional) {
     // EXPECT_EQ(*optional_base_ref, derived(3,4));
 
 }
+
+TEST(OptionalRefTest, ConstructFromOptional) {
+    int                               var = 42;
+    beman::optional26::optional<int&> o1  = beman::optional26::nullopt;
+    beman::optional26::optional<int&> o2{var};
+
+
+    using beman::optional26::tests::base;
+    using beman::optional26::tests::derived;
+
+    base                               b{1};
+    derived                            d(1, 2);
+    beman::optional26::optional<base&> empty_base;
+    beman::optional26::optional<base&> engaged_base{b};
+
+    beman::optional26::optional<derived&> empty_derived_ref;
+    beman::optional26::optional<derived&> engaged_derived_ref{d};
+
+    beman::optional26::optional<base&> optional_base_ref{empty_derived_ref};
+    EXPECT_FALSE(optional_base_ref.has_value());
+
+    beman::optional26::optional<base&> optional_base_ref2{engaged_derived_ref};
+    EXPECT_TRUE(optional_base_ref2.has_value());
+
+    beman::optional26::optional<derived> empty_derived;
+    beman::optional26::optional<derived> engaged_derived{d};
+
+    static_assert(std::is_constructible_v<const base&, derived>);
+
+    beman::optional26::optional<const base&> optional_base_const_ref{empty_derived};
+    EXPECT_FALSE(optional_base_const_ref.has_value());
+
+    beman::optional26::optional<const base&> optional_base_const_ref2{engaged_derived};
+    EXPECT_TRUE(optional_base_const_ref2.has_value());
+
+}