fix: compile error when deserializing into types using ordered_map

docs/mkdocs/docs/home/faq.md

@@ -20,7 +20,16 @@
 
     yield different results (`#!json [true]` vs. `#!json true`)?
 
-This is a known issue, and -- even worse -- the behavior differs between GCC and Clang. The "culprit" for this is the library's constructor overloads for initializer lists to allow syntax like
+Starting from this version, single-value brace initialization is treated as copy/move instead of wrapping in a single-element array:
+
+```cpp
+json j_orig = {{"key", "value"}};
+
+json j{j_orig};
+j_orig.method();
+```
+
+The root cause was the library's constructor overloads for initializer lists, which allow syntax like
 
 ```cpp
 json array = {1, 2, 3, 4};
@@ -32,11 +41,13 @@ for arrays and
 json object = {{"one", 1}, {"two", 2}}; 
 ```
 
-for objects.
+for objects. Because C++ always prefers `initializer_list` constructors for brace initialization, a single-element brace-init `json j{someValue}` previously wrapped the value in an array instead of copying it.
 
-!!! tip
+After the fix, a single-element brace-init behaves like copy/move initialization. To explicitly create a single-element array, use `json::array({value})`.
+
+!!! note
 
-    To avoid any confusion and ensure portable code, **do not** use brace initialization with the types `basic_json`, `json`, or `ordered_json` unless you want to create an object or array as shown in the examples above.
+The fix changes the behavior of single-element brace initialization: `json j{x}` is now equivalent to `json j(x)` (copy/move) rather than `json j = json::array({x})` (single-element array). Code relying on the old behavior should be updated to use `json::array({x})` explicitly.
 
 ## Limitations
 

include/nlohmann/detail/conversions/from_json.hpp

@@ -386,14 +386,10 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 
     ConstructibleObjectType ret;
     const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    using value_type = typename ConstructibleObjectType::value_type;
-    std::transform(
-        inner_object->begin(), inner_object->end(),
-        std::inserter(ret, ret.begin()),
-        [](typename BasicJsonType::object_t::value_type const & p)
+    for (const auto& p : *inner_object)
     {
-        return value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    });
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
     obj = std::move(ret);
 }
 

include/nlohmann/json.hpp

@@ -955,6 +955,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
         else
         {
+            // if there is exactly one element and type deduction is enabled,
+            // treat it as copy/move to avoid unintentionally wrapping a single
+            // json value in a single-element array
+            // (see https://github.com/nlohmann/json/issues/5074)
+            if (type_deduction && init.size() == 1)
+            {
+                *this = init.begin()->moved_or_copied();
+                set_parents();
+                assert_invariant();
+                return;
+            }
+
             // the initializer list describes an array -> create an array
             m_data.m_type = value_t::array;
             m_data.m_value.array = create<array_t>(init.begin(), init.end());

single_include/nlohmann/json.hpp

@@ -3499,71 +3499,71 @@ NLOHMANN_JSON_NAMESPACE_END
 // SPDX-License-Identifier: MIT
 
 #ifndef INCLUDE_NLOHMANN_JSON_FWD_HPP_
-    #define INCLUDE_NLOHMANN_JSON_FWD_HPP_
+#define INCLUDE_NLOHMANN_JSON_FWD_HPP_
 
-    #include <cstdint> // int64_t, uint64_t
-    #include <map> // map
-    #include <memory> // allocator
-    #include <string> // string
-    #include <vector> // vector
+#include <cstdint> // int64_t, uint64_t
+#include <map> // map
+#include <memory> // allocator
+#include <string> // string
+#include <vector> // vector
 
-    // #include <nlohmann/detail/abi_macros.hpp>
+// #include <nlohmann/detail/abi_macros.hpp>
 
 
-    /*!
-    @brief namespace for Niels Lohmann
-    @see https://github.com/nlohmann
-    @since version 1.0.0
-    */
-    NLOHMANN_JSON_NAMESPACE_BEGIN
+/*!
+@brief namespace for Niels Lohmann
+@see https://github.com/nlohmann
+@since version 1.0.0
+*/
+NLOHMANN_JSON_NAMESPACE_BEGIN
 
-    /*!
-    @brief default JSONSerializer template argument
+/*!
+@brief default JSONSerializer template argument
 
-    This serializer ignores the template arguments and uses ADL
-    ([argument-dependent lookup](https://en.cppreference.com/w/cpp/language/adl))
-    for serialization.
-    */
-    template<typename T = void, typename SFINAE = void>
-    struct adl_serializer;
-
-    /// a class to store JSON values
-    /// @sa https://json.nlohmann.me/api/basic_json/
-    template<template<typename U, typename V, typename... Args> class ObjectType =
-    std::map,
-    template<typename U, typename... Args> class ArrayType = std::vector,
-    class StringType = std::string, class BooleanType = bool,
-    class NumberIntegerType = std::int64_t,
-    class NumberUnsignedType = std::uint64_t,
-    class NumberFloatType = double,
-    template<typename U> class AllocatorType = std::allocator,
-    template<typename T, typename SFINAE = void> class JSONSerializer =
-    adl_serializer,
-    class BinaryType = std::vector<std::uint8_t>, // cppcheck-suppress syntaxError
-    class CustomBaseClass = void>
-    class basic_json;
-
-    /// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
-    /// @sa https://json.nlohmann.me/api/json_pointer/
-    template<typename RefStringType>
-    class json_pointer;
+This serializer ignores the template arguments and uses ADL
+([argument-dependent lookup](https://en.cppreference.com/w/cpp/language/adl))
+for serialization.
+*/
+template<typename T = void, typename SFINAE = void>
+struct adl_serializer;
+
+/// a class to store JSON values
+/// @sa https://json.nlohmann.me/api/basic_json/
+template<template<typename U, typename V, typename... Args> class ObjectType =
+         std::map,
+         template<typename U, typename... Args> class ArrayType = std::vector,
+         class StringType = std::string, class BooleanType = bool,
+         class NumberIntegerType = std::int64_t,
+         class NumberUnsignedType = std::uint64_t,
+         class NumberFloatType = double,
+         template<typename U> class AllocatorType = std::allocator,
+         template<typename T, typename SFINAE = void> class JSONSerializer =
+         adl_serializer,
+         class BinaryType = std::vector<std::uint8_t>, // cppcheck-suppress syntaxError
+         class CustomBaseClass = void>
+class basic_json;
 
-    /*!
-    @brief default specialization
-    @sa https://json.nlohmann.me/api/json/
-    */
-    using json = basic_json<>;
+/// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
+/// @sa https://json.nlohmann.me/api/json_pointer/
+template<typename RefStringType>
+class json_pointer;
 
-    /// @brief a minimal map-like container that preserves insertion order
-    /// @sa https://json.nlohmann.me/api/ordered_map/
-    template<class Key, class T, class IgnoredLess, class Allocator>
-    struct ordered_map;
+/*!
+@brief default specialization
+@sa https://json.nlohmann.me/api/json/
+*/
+using json = basic_json<>;
+
+/// @brief a minimal map-like container that preserves insertion order
+/// @sa https://json.nlohmann.me/api/ordered_map/
+template<class Key, class T, class IgnoredLess, class Allocator>
+struct ordered_map;
 
-    /// @brief specialization that maintains the insertion order of object keys
-    /// @sa https://json.nlohmann.me/api/ordered_json/
-    using ordered_json = basic_json<nlohmann::ordered_map>;
+/// @brief specialization that maintains the insertion order of object keys
+/// @sa https://json.nlohmann.me/api/ordered_json/
+using ordered_json = basic_json<nlohmann::ordered_map>;
 
-    NLOHMANN_JSON_NAMESPACE_END
+NLOHMANN_JSON_NAMESPACE_END
 
 #endif  // INCLUDE_NLOHMANN_JSON_FWD_HPP_
 
@@ -5219,14 +5219,10 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 
     ConstructibleObjectType ret;
     const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    using value_type = typename ConstructibleObjectType::value_type;
-    std::transform(
-        inner_object->begin(), inner_object->end(),
-        std::inserter(ret, ret.begin()),
-        [](typename BasicJsonType::object_t::value_type const & p)
+    for (const auto& p : *inner_object)
     {
-        return value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    });
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
     obj = std::move(ret);
 }
 
@@ -5427,7 +5423,7 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 // #include <nlohmann/detail/macro_scope.hpp>
-// JSON_HAS_CPP_17
+ // JSON_HAS_CPP_17
 #ifdef JSON_HAS_CPP_17
     #include <optional> // optional
 #endif
@@ -20255,10 +20251,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         const bool allow_exceptions = true,
         const bool ignore_comments = false,
         const bool ignore_trailing_commas = false
-                                 )
+    )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
-            std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
+               std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
     }
 
   private:
@@ -20956,8 +20952,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                detail::enable_if_t <
                    !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value, int > = 0 >
     basic_json(CompatibleType && val) noexcept(noexcept( // NOLINT(bugprone-forwarding-reference-overload,bugprone-exception-escape)
-            JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
-                                       std::forward<CompatibleType>(val))))
+                JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
+                                           std::forward<CompatibleType>(val))))
     {
         JSONSerializer<U>::to_json(*this, std::forward<CompatibleType>(val));
         set_parents();
@@ -21074,6 +21070,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
         else
         {
+            // if there is exactly one element and type deduction is enabled,
+            // treat it as copy/move to avoid unintentionally wrapping a single
+            // json value in a single-element array
+            // (see https://github.com/nlohmann/json/issues/5074)
+            if (type_deduction && init.size() == 1)
+            {
+                *this = init.begin()->moved_or_copied();
+                set_parents();
+                assert_invariant();
+                return;
+            }
+
             // the initializer list describes an array -> create an array
             m_data.m_type = value_t::array;
             m_data.m_value.array = create<array_t>(init.begin(), init.end());
@@ -21751,7 +21759,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    detail::has_from_json<basic_json_t, ValueType>::value,
                    int > = 0 >
     ValueType get_impl(detail::priority_tag<0> /*unused*/) const noexcept(noexcept(
-            JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
     {
         auto ret = ValueType();
         JSONSerializer<ValueType>::from_json(*this, ret);
@@ -21793,7 +21801,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    detail::has_non_default_from_json<basic_json_t, ValueType>::value,
                    int > = 0 >
     ValueType get_impl(detail::priority_tag<1> /*unused*/) const noexcept(noexcept(
-            JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>())))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>())))
     {
         return JSONSerializer<ValueType>::from_json(*this);
     }
@@ -21943,7 +21951,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    detail::has_from_json<basic_json_t, ValueType>::value,
                    int > = 0 >
     ValueType & get_to(ValueType& v) const noexcept(noexcept(
-            JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), v)))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), v)))
     {
         JSONSerializer<ValueType>::from_json(*this, v);
         return v;

tests/src/unit-class_parser.cpp

@@ -1506,7 +1506,7 @@ TEST_CASE("parser class")
 
             // removed all objects in array.
             CHECK (j_filtered2.size() == 1);
-            CHECK (j_filtered2 == json({1}));
+            CHECK (j_filtered2 == json::array({1}));
         }
 
         SECTION("filter specific events")

tests/src/unit-class_parser_diagnostic_positions.cpp

@@ -1517,7 +1517,7 @@ TEST_CASE("parser class")
 
             // removed all objects in array.
             CHECK (j_filtered2.size() == 1);
-            CHECK (j_filtered2 == json({1}));
+            CHECK (j_filtered2 == json::array({1}));
         }
 
         SECTION("filter specific events")