Fix the CI failures of the MessagePack length check

- mark to_msgpack_length's value as used when exceptions are disabled
  (-Wunused-parameter, misc-unused-parameters)
- put "Exception safety" before "Exceptions" in to_msgpack.md, as the
  documentation style check requires
- create the test's string value from its type: constructing it from a
  beyond_uint32_string_t considers the std::filesystem::path conversion,
  which libstdc++ 10 reports as ambiguous for a class derived from
  std::string (clang 13)

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

docs/mkdocs/docs/api/basic_json/to_msgpack.md

@@ -30,6 +30,10 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 1. MessagePack serialization as a byte vector
 2. (none)
 
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
+
 ## Exceptions
 
 - Throws [`out_of_range.412`](../../home/exceptions.md#jsonexceptionout_of_range412) if the length of a string, binary
@@ -39,10 +43,6 @@ The exact mapping and its limitations are described on a [dedicated page](../../
   exceeds 255, the maximum of the MessagePack ext type; example:
   `"subtype 70000 is too large for the MessagePack ext type (max 255)"`
 
-## Exception safety
-
-Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
-
 ## Complexity
 
 Linear in the size of the JSON value `j`.

include/nlohmann/detail/output/binary_writer.hpp

@@ -479,6 +479,7 @@ class binary_writer
             JSON_THROW(out_of_range::create(412, concat("MessagePack length ", std::to_string(length), " exceeds maximum of ", std::to_string((std::numeric_limits<std::uint32_t>::max)())), &j));
         }
 
+        static_cast<void>(j);
         return static_cast<std::uint32_t>(length);
     }
 

single_include/nlohmann/json.hpp

@@ -19944,6 +19944,7 @@ class binary_writer
             JSON_THROW(out_of_range::create(412, concat("MessagePack length ", std::to_string(length), " exceeds maximum of ", std::to_string((std::numeric_limits<std::uint32_t>::max)())), &j));
         }
 
+        static_cast<void>(j);
         return static_cast<std::uint32_t>(length);
     }
 

tests/src/unit-msgpack.cpp

@@ -2208,7 +2208,10 @@ TEST_CASE("MessagePack lengths beyond UINT32_MAX cannot be serialized")
         const beyond_uint32_binary_json ext = beyond_uint32_binary_json::binary(beyond_uint32_binary_t{}, 42);
         CHECK_THROWS_WITH_AS(beyond_uint32_binary_json::to_msgpack(ext), expected, beyond_uint32_binary_json::out_of_range&);
 
-        const beyond_uint32_string_json string = beyond_uint32_string_t("value");
+        // created from its type rather than from a beyond_uint32_string_t:
+        // that would consider the std::filesystem::path conversion, which
+        // libstdc++ 10 cannot decide for a class derived from std::string
+        const beyond_uint32_string_json string(beyond_uint32_string_json::value_t::string);
         CHECK_THROWS_WITH_AS(beyond_uint32_string_json::to_msgpack(string), expected, beyond_uint32_string_json::out_of_range&);
     }
 #endif