Added tests for bug when swapping value/null-value and null-value value

tests/swap.cpp

@@ -0,0 +1,26 @@
+#include "catch.hpp"
+#include "optional.hpp"
+
+TEST_CASE("Swap value", "[swap.value]") {
+  tl::optional<int> o1 = 42;
+  tl::optional<int> o2 = 12;
+  o1.swap(o2);
+  CHECK(o1.value() == 12);
+  CHECK(o2.value() == 42);
+}
+
+TEST_CASE("Swap value with null intialized", "[swap.value_nullopt]") {
+  tl::optional<int> o1 = 42;
+  tl::optional<int> o2 = tl::nullopt;
+  o1.swap(o2);
+  CHECK(!o1.has_value());
+  CHECK(o2.value() == 42);
+}
+
+TEST_CASE("Swap null intialized with value", "[swap.nullopt_value]") {
+  tl::optional<int> o1 = tl::nullopt;
+  tl::optional<int> o2 = 42;
+  o1.swap(o2);
+  CHECK(o1.value() == 42);
+  CHECK(!o2.has_value());
+}