Fix CI: clang-tidy and clang/libstdc++ 10 in the MessagePack size tests (#5599)

The tests added by #5515 fail two ways on develop:

- clang-tidy reports the size() overrides of huge_string and huge_binary
  (readability-convert-member-functions-to-static) and the non-const
  test value (misc-const-correctness); mark them like the #5584 types
- clang with libstdc++ 10 cannot compile the file for C++17: the
  std::filesystem::path conversion considered for huge_string, a class
  derived from std::string, is ambiguous. Guard it with
  JSON_TEST_BEYOND_UINT32_STRING, which #5584 introduced for the same
  reason, and define that macro before both test blocks.

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

tests/src/unit-msgpack.cpp

@@ -2165,6 +2165,13 @@ TEST_CASE("MessagePack with std::byte")
 #endif
 
 // the fake sizes below do not fit into a 32-bit std::size_t
+// with clang and libstdc++ 10, the std::filesystem::path conversion that
+// C++17 builds consider for every string type is ambiguous for a class
+// derived from std::string, so the string case is not tested there
+#if !(defined(__clang__) && defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 11)
+    #define JSON_TEST_BEYOND_UINT32_STRING 1
+#endif
+
 #if SIZE_MAX > UINT32_MAX
 template<typename T, typename A = std::allocator<T>>
 struct huge_array : std::vector<T, A>
@@ -2262,11 +2269,12 @@ TEST_CASE("MessagePack Size above uint32 for object")
     object.fake_size = false;
 }
 
+#ifdef JSON_TEST_BEYOND_UINT32_STRING
 struct huge_string : std::string
 {
     using std::string::string;
 
-    std::size_t size() const noexcept
+    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
     {
         return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
     }
@@ -2287,20 +2295,20 @@ using huge_string_json = nlohmann::basic_json <
 
 TEST_CASE("MessagePack Size above uint32 for string")
 {
-
-    huge_string_json j = "hello";
+    const huge_string_json j = "hello";
 
     CHECK_THROWS_WITH_AS(
         huge_string_json::to_msgpack(j),
         "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
         json::out_of_range&);
 }
+#endif
 
 struct huge_binary : std::vector<std::uint8_t>
 {
     using std::vector<std::uint8_t>::vector;
 
-    std::size_t size() const noexcept
+    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
     {
         return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
     }
@@ -2355,13 +2363,6 @@ class beyond_uint32_binary_t : public std::vector<std::uint8_t>
     }
 };
 
-// with clang and libstdc++ 10, the std::filesystem::path conversion that
-// C++17 builds consider for every string type is ambiguous for a class
-// derived from std::string, so the string case is not tested there
-#if !(defined(__clang__) && defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 11)
-    #define JSON_TEST_BEYOND_UINT32_STRING 1
-#endif
-
 #ifdef JSON_TEST_BEYOND_UINT32_STRING
 class beyond_uint32_string_t : public std::string
 {