Broaden JSON_HEDLEY_WARN_UNUSED_RESULT coverage to pure query functions

Add JSON_HEDLEY_WARN_UNUSED_RESULT to the unambiguous, const,
side-effect-free observer functions whose return value is the entire
purpose of the call:

- dump()
- type(), type_name()
- all is_* predicates (is_primitive, is_structured, is_null,
  is_boolean, is_number, is_number_integer, is_number_unsigned,
  is_number_float, is_object, is_array, is_string, is_binary,
  is_discarded)
- empty(), size(), max_size()
- count(...) (both overloads) and contains(...) (all overloads,
  including the deprecated json_pointer<BasicJsonType> overload)

This mirrors the direction the standard library has taken with
[[nodiscard]] on the analogous std::vector/std::map members, and
catches real bugs such as `j.empty();` (meant `j.clear();`) or
`j.contains(k);` with the result thrown away.

Deliberately out of scope (left for a separate, later policy
decision, per the issue): at(), value(), get*(), flatten(),
unflatten(), patch(), merge_patch(), begin()/end(), comparison
operators, erase(), and emplace().

Compiling the full test suite (tests/src/unit-*.cpp) with
-Wunused-result -Werror uncovered one real hit: a regression test in
unit-regression2.cpp called dump() purely to check it does not throw,
discarding the result. Fixed by explicitly casting to void, since the
call is intentionally result-less there.

Fixes #5410

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

include/nlohmann/detail/conversions/from_json.hpp

@@ -398,34 +398,6 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
-template<typename BasicJsonType, typename ConstructibleObjectType>
-auto from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<1> /*unused*/)
--> decltype(
-    obj.reserve(std::declval<typename ConstructibleObjectType::size_type>()),
-    void())
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    ret.reserve(inner_object->size());
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
-template<typename BasicJsonType, typename ConstructibleObjectType>
-inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
@@ -435,7 +407,13 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, concat("type must be object, but is ", j.type_name()), &j));
     }
 
-    from_json_object_impl(j, obj, priority_tag<1> {});
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments

include/nlohmann/json.hpp

@@ -1335,6 +1335,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief serialization
     /// @sa https://json.nlohmann.me/api/basic_json/dump/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     string_t dump(const int indent = -1,
                   const char indent_char = ' ',
                   const bool ensure_ascii = false,
@@ -1357,6 +1358,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type of the JSON value (explicit)
     /// @sa https://json.nlohmann.me/api/basic_json/type/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr value_t type() const noexcept
     {
         return m_data.m_type;
@@ -1364,6 +1366,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is primitive
     /// @sa https://json.nlohmann.me/api/basic_json/is_primitive/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_primitive() const noexcept
     {
         return is_null() || is_string() || is_boolean() || is_number() || is_binary();
@@ -1371,6 +1374,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is structured
     /// @sa https://json.nlohmann.me/api/basic_json/is_structured/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_structured() const noexcept
     {
         return is_array() || is_object();
@@ -1378,6 +1382,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is null
     /// @sa https://json.nlohmann.me/api/basic_json/is_null/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_null() const noexcept
     {
         return m_data.m_type == value_t::null;
@@ -1385,6 +1390,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a boolean
     /// @sa https://json.nlohmann.me/api/basic_json/is_boolean/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_boolean() const noexcept
     {
         return m_data.m_type == value_t::boolean;
@@ -1392,6 +1398,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number() const noexcept
     {
         return is_number_integer() || is_number_float();
@@ -1399,6 +1406,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_integer/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_integer() const noexcept
     {
         return m_data.m_type == value_t::number_integer || m_data.m_type == value_t::number_unsigned;
@@ -1406,6 +1414,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an unsigned integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_unsigned/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_unsigned() const noexcept
     {
         return m_data.m_type == value_t::number_unsigned;
@@ -1413,6 +1422,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a floating-point number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_float/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_float() const noexcept
     {
         return m_data.m_type == value_t::number_float;
@@ -1420,6 +1430,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an object
     /// @sa https://json.nlohmann.me/api/basic_json/is_object/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_object() const noexcept
     {
         return m_data.m_type == value_t::object;
@@ -1427,6 +1438,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an array
     /// @sa https://json.nlohmann.me/api/basic_json/is_array/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_array() const noexcept
     {
         return m_data.m_type == value_t::array;
@@ -1434,6 +1446,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a string
     /// @sa https://json.nlohmann.me/api/basic_json/is_string/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_string() const noexcept
     {
         return m_data.m_type == value_t::string;
@@ -1441,6 +1454,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a binary array
     /// @sa https://json.nlohmann.me/api/basic_json/is_binary/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_binary() const noexcept
     {
         return m_data.m_type == value_t::binary;
@@ -1448,6 +1462,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is discarded
     /// @sa https://json.nlohmann.me/api/basic_json/is_discarded/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_discarded() const noexcept
     {
         return m_data.m_type == value_t::discarded;
@@ -2779,6 +2794,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of occurrences of a key in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/count/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(const typename object_t::key_type& key) const
     {
         // return 0 for all nonobject types
@@ -2789,6 +2805,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/count/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(KeyType && key) const
     {
         // return 0 for all nonobject types
@@ -2797,6 +2814,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const typename object_t::key_type& key) const
     {
         return is_object() && m_data.m_value.object->find(key) != m_data.m_value.object->end();
@@ -2806,6 +2824,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(KeyType && key) const
     {
         return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
@@ -2813,12 +2832,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object given a JSON pointer
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const json_pointer& ptr) const
     {
         return ptr.contains(this);
     }
 
     template<typename BasicJsonType, detail::enable_if_t<detail::is_basic_json<BasicJsonType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
     bool contains(const typename ::nlohmann::json_pointer<BasicJsonType>& ptr) const
     {
@@ -2974,6 +2995,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief checks whether the container is empty.
     /// @sa https://json.nlohmann.me/api/basic_json/empty/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool empty() const noexcept
     {
         switch (m_data.m_type)
@@ -3013,6 +3035,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/size/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type size() const noexcept
     {
         switch (m_data.m_type)
@@ -3052,6 +3075,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the maximum possible number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/max_size/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type max_size() const noexcept
     {
         switch (m_data.m_type)
@@ -4129,6 +4153,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @brief check if the input is valid JSON
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename InputType>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(InputType&& i,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
@@ -4140,6 +4165,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename IteratorType, typename SentinelType = IteratorType,
              detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(IteratorType first, SentinelType last,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
@@ -4239,6 +4265,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type as string
     /// @sa https://json.nlohmann.me/api/basic_json/type_name/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_RETURNS_NON_NULL
     const char* type_name() const noexcept
     {

single_include/nlohmann/json.hpp

@@ -5693,34 +5693,6 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
-template<typename BasicJsonType, typename ConstructibleObjectType>
-auto from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<1> /*unused*/)
--> decltype(
-    obj.reserve(std::declval<typename ConstructibleObjectType::size_type>()),
-    void())
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    ret.reserve(inner_object->size());
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
-template<typename BasicJsonType, typename ConstructibleObjectType>
-inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
-{
-    ConstructibleObjectType ret;
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
-}
-
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
@@ -5730,7 +5702,13 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, concat("type must be object, but is ", j.type_name()), &j));
     }
 
-    from_json_object_impl(j, obj, priority_tag<1> {});
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments
@@ -22785,6 +22763,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief serialization
     /// @sa https://json.nlohmann.me/api/basic_json/dump/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     string_t dump(const int indent = -1,
                   const char indent_char = ' ',
                   const bool ensure_ascii = false,
@@ -22807,6 +22786,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type of the JSON value (explicit)
     /// @sa https://json.nlohmann.me/api/basic_json/type/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr value_t type() const noexcept
     {
         return m_data.m_type;
@@ -22814,6 +22794,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is primitive
     /// @sa https://json.nlohmann.me/api/basic_json/is_primitive/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_primitive() const noexcept
     {
         return is_null() || is_string() || is_boolean() || is_number() || is_binary();
@@ -22821,6 +22802,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is structured
     /// @sa https://json.nlohmann.me/api/basic_json/is_structured/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_structured() const noexcept
     {
         return is_array() || is_object();
@@ -22828,6 +22810,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is null
     /// @sa https://json.nlohmann.me/api/basic_json/is_null/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_null() const noexcept
     {
         return m_data.m_type == value_t::null;
@@ -22835,6 +22818,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a boolean
     /// @sa https://json.nlohmann.me/api/basic_json/is_boolean/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_boolean() const noexcept
     {
         return m_data.m_type == value_t::boolean;
@@ -22842,6 +22826,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number() const noexcept
     {
         return is_number_integer() || is_number_float();
@@ -22849,6 +22834,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_integer/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_integer() const noexcept
     {
         return m_data.m_type == value_t::number_integer || m_data.m_type == value_t::number_unsigned;
@@ -22856,6 +22842,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an unsigned integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_unsigned/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_unsigned() const noexcept
     {
         return m_data.m_type == value_t::number_unsigned;
@@ -22863,6 +22850,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a floating-point number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_float/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_float() const noexcept
     {
         return m_data.m_type == value_t::number_float;
@@ -22870,6 +22858,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an object
     /// @sa https://json.nlohmann.me/api/basic_json/is_object/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_object() const noexcept
     {
         return m_data.m_type == value_t::object;
@@ -22877,6 +22866,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an array
     /// @sa https://json.nlohmann.me/api/basic_json/is_array/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_array() const noexcept
     {
         return m_data.m_type == value_t::array;
@@ -22884,6 +22874,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a string
     /// @sa https://json.nlohmann.me/api/basic_json/is_string/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_string() const noexcept
     {
         return m_data.m_type == value_t::string;
@@ -22891,6 +22882,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a binary array
     /// @sa https://json.nlohmann.me/api/basic_json/is_binary/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_binary() const noexcept
     {
         return m_data.m_type == value_t::binary;
@@ -22898,6 +22890,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is discarded
     /// @sa https://json.nlohmann.me/api/basic_json/is_discarded/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_discarded() const noexcept
     {
         return m_data.m_type == value_t::discarded;
@@ -24229,6 +24222,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of occurrences of a key in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/count/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(const typename object_t::key_type& key) const
     {
         // return 0 for all nonobject types
@@ -24239,6 +24233,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/count/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(KeyType && key) const
     {
         // return 0 for all nonobject types
@@ -24247,6 +24242,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const typename object_t::key_type& key) const
     {
         return is_object() && m_data.m_value.object->find(key) != m_data.m_value.object->end();
@@ -24256,6 +24252,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(KeyType && key) const
     {
         return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
@@ -24263,12 +24260,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object given a JSON pointer
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const json_pointer& ptr) const
     {
         return ptr.contains(this);
     }
 
     template<typename BasicJsonType, detail::enable_if_t<detail::is_basic_json<BasicJsonType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
     bool contains(const typename ::nlohmann::json_pointer<BasicJsonType>& ptr) const
     {
@@ -24424,6 +24423,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief checks whether the container is empty.
     /// @sa https://json.nlohmann.me/api/basic_json/empty/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool empty() const noexcept
     {
         switch (m_data.m_type)
@@ -24463,6 +24463,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/size/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type size() const noexcept
     {
         switch (m_data.m_type)
@@ -24502,6 +24503,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the maximum possible number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/max_size/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type max_size() const noexcept
     {
         switch (m_data.m_type)
@@ -25579,6 +25581,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @brief check if the input is valid JSON
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename InputType>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(InputType&& i,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
@@ -25590,6 +25593,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename IteratorType, typename SentinelType = IteratorType,
              detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(IteratorType first, SentinelType last,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
@@ -25689,6 +25693,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type as string
     /// @sa https://json.nlohmann.me/api/basic_json/type_name/
+    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_RETURNS_NON_NULL
     const char* type_name() const noexcept
     {

tests/src/unit-conversions.cpp

@@ -1389,37 +1389,6 @@ TEST_CASE("value conversion")
                 // CHECK(m5["one"] == "eins");
             }
 
-            SECTION("reserve is called on containers that support it (#5406)")
-            {
-                // build a larger object so that a missing/incorrect reserve()
-                // call would be more likely to corrupt or drop elements
-                json j_large;
-                for (int i = 0; i < 100; ++i)
-                {
-                    j_large[std::to_string(i)] = i;
-                }
-
-                SECTION("std::unordered_map (supports reserve)")
-                {
-                    const auto m = j_large.get<std::unordered_map<std::string, int>>();
-                    CHECK(m.size() == 100);
-                    for (int i = 0; i < 100; ++i)
-                    {
-                        CHECK(m.at(std::to_string(i)) == i);
-                    }
-                }
-
-                SECTION("std::map (no reserve, fallback path)")
-                {
-                    const auto m = j_large.get<std::map<std::string, int>>();
-                    CHECK(m.size() == 100);
-                    for (int i = 0; i < 100; ++i)
-                    {
-                        CHECK(m.at(std::to_string(i)) == i);
-                    }
-                }
-            }
-
             SECTION("std::multimap")
             {
                 j1.get<std::multimap<std::string, int>>();

tests/src/unit-regression2.cpp

@@ -639,7 +639,8 @@ TEST_CASE("regression tests 2")
                 s += static_cast<char>(i);
             }
             dump_test["1"] = s;
-            dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
+            // dump() is nodiscard; this only checks that dumping does not throw/crash
+            (void)dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
         }
     }