test: verify move semantics of byte_container_with_subtype's rvalue constructors (#5423)

The two rvalue-reference constructors were never distinguished from their
const-lvalue-reference twins by any test. Add a "move semantics" section that
constructs from an rvalue std::vector, checks the resulting container keeps
the exact same buffer address as the source (a stronger check than just
observing the source ended up empty, since a copy-then-clear could do that
too), and confirms the source vector was left empty.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-byte_container_with_subtype.cpp

@@ -42,6 +42,39 @@ TEST_CASE("byte_container_with_subtype")
         CHECK(container.subtype() == static_cast<subtype_type>(-1));
     }
 
+    SECTION("move semantics")
+    {
+        // the rvalue-reference constructor (without a subtype) must actually move
+        // the passed-in container rather than copy it; comparing the buffer address
+        // before and after is a stronger check than just observing the source is
+        // empty afterward, since a copy-then-clear could also leave it empty
+        {
+            std::vector<std::uint8_t> bytes = {{0xCA, 0xFE, 0xBA, 0xBE}};
+            const auto* const data_ptr = bytes.data();
+
+            nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>> container(std::move(bytes));
+
+            CHECK(container.size() == 4);
+            CHECK(container.data() == data_ptr);
+            CHECK(!container.has_subtype());
+            CHECK(bytes.empty()); // NOLINT(bugprone-use-after-move,clang-analyzer-cplusplus.Move,hicpp-invalid-access-moved)
+        }
+
+        // same check for the rvalue-reference constructor that also takes a subtype
+        {
+            std::vector<std::uint8_t> bytes = {{0xCA, 0xFE, 0xBA, 0xBE}};
+            const auto* const data_ptr = bytes.data();
+
+            nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>> container(std::move(bytes), 42);
+
+            CHECK(container.size() == 4);
+            CHECK(container.data() == data_ptr);
+            CHECK(container.has_subtype());
+            CHECK(container.subtype() == 42);
+            CHECK(bytes.empty()); // NOLINT(bugprone-use-after-move,clang-analyzer-cplusplus.Move,hicpp-invalid-access-moved)
+        }
+    }
+
     SECTION("comparisons")
     {
         std::vector<std::uint8_t> const bytes = {{0xCA, 0xFE, 0xBA, 0xBE}};