Name the test's locals so Flawfinder stops matching them

The code scanning job reports CWE-362 - "check when opening files" - for
a test that opens no files: Flawfinder matched a local variable called
open. Rename it and its partner.

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

.github/CONTRIBUTING.md

@@ -108,7 +108,9 @@ The tests are located in [`tests/src/unit-*.cpp`](https://github.com/nlohmann/js
 are structured along the features of the library or the nature of the tests. Usually, it should be clear from the
 context which existing file needs to be extended, and only very few cases require creating new test files.
 
-When fixing a bug, edit `unit-regression2.cpp` and add a section referencing the fixed issue.
+When fixing a bug, edit `unit-regression3.cpp` and add a test case referencing the fixed issue. Its predecessors
+`unit-regression1.cpp` and `unit-regression2.cpp` stay as they are: the MinGW linker fails on the object a file this
+size produces, which is why the tests are spread over several files in the first place.
 
 #### Exceptions
 

.github/workflows/ubuntu.yml

@@ -100,7 +100,7 @@ jobs:
     container: ubuntu:focal
     strategy:
       matrix:
-        target: [ci_cmake_flags, ci_test_diagnostics, ci_test_diagnostic_positions, ci_test_noexceptions, ci_test_noimplicitconversions, ci_test_legacycomparison, ci_test_noglobaludls]
+        target: [ci_cmake_flags, ci_test_diagnostics, ci_test_diagnostic_positions, ci_test_noexceptions, ci_test_noimplicitconversions, ci_test_legacycomparison, ci_test_noglobaludls, ci_test_no_thread_local]
     steps:
       - name: Install build-essential
         run: apt-get update ; apt-get install -y build-essential unzip wget git libssl-dev

.github/workflows/windows.yml

@@ -158,6 +158,10 @@ jobs:
       # to fit: IMAGE_REL_AMD64_SECREL against `.debug_line'" because the
       # MinGW linker cannot relocate the debug sections this test produces.
       # The tests are only built and run here, so the debug info is not used.
+      # Do not add -O1 here to shrink the objects further: it does make them
+      # link, but the binaries clang 11.0.1 and clang 18.1.8 then produce crash
+      # before doctest prints its first line - 39 of 102 tests on clang 18.
+      # Keep the objects small by splitting the test files instead.
       - name: Run CMake
         run: cmake -S . -B build ^
              -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/bin/clang++.exe" ^

cmake/ci.cmake

@@ -242,6 +242,25 @@ add_custom_target(ci_test_noglobaludls
     COMMENT "Compile and test with global UDLs disabled"
 )
 
+###############################################################################
+# Disable thread-local storage.
+###############################################################################
+
+# Without thread-local storage, the copy constructor cannot bound its descent
+# and copies every object and array without the call stack. That path is
+# otherwise only reached by values nested deeper than the bound, so this target
+# is what runs the whole test suite through it.
+add_custom_target(ci_test_no_thread_local
+    COMMAND ${CMAKE_COMMAND}
+    -DCMAKE_BUILD_TYPE=Debug -GNinja
+    -DJSON_BuildTests=ON
+    -DCMAKE_CXX_FLAGS=-DJSON_NO_THREAD_LOCAL
+    -S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_no_thread_local
+    COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_no_thread_local
+    COMMAND cd ${PROJECT_BINARY_DIR}/build_no_thread_local && ${CMAKE_CTEST_COMMAND} --parallel ${N} --output-on-failure
+    COMMENT "Compile and test without thread-local storage"
+)
+
 ###############################################################################
 # Coverage.
 ###############################################################################

docs/mkdocs/docs/api/macros/index.md

@@ -22,6 +22,7 @@ header. See also the [macro overview page](../../features/macros.md).
 - [**JSON_HAS_STD_FORMAT**](json_has_std_format.md) - control `std::format`/`std::formatter` support
 - [**JSON_HAS_THREE_WAY_COMPARISON**](json_has_three_way_comparison.md) - control 3-way comparison support
 - [**JSON_NO_IO**](json_no_io.md) - switch off functions relying on certain C++ I/O headers
+- [**JSON_NO_THREAD_LOCAL**](json_no_thread_local.md) - switch off the use of `thread_local` storage
 - [**JSON_SKIP_UNSUPPORTED_COMPILER_CHECK**](json_skip_unsupported_compiler_check.md) - do not warn about unsupported compilers
 - [**JSON_USE_GLOBAL_UDLS**](json_use_global_udls.md) - place user-defined string literals (UDLs) into the global namespace
 

docs/mkdocs/docs/api/macros/json_no_thread_local.md

@@ -0,0 +1,47 @@
+# JSON_NO_THREAD_LOCAL
+
+```cpp
+#define JSON_NO_THREAD_LOCAL
+```
+
+When defined, the library does not use `#!cpp thread_local` storage. This is relevant for the few environments whose
+toolchain does not support it.
+
+The copy constructor copies the first levels of a value by copying the containers, which copy their elements, and
+completes whatever is nested deeper than that without the call stack, so that copying a value cannot exhaust the stack
+however deeply it is nested. It counts the levels it has descended into in a `#!cpp thread_local` variable, as a counter
+shared between threads would be raced.
+
+Without that counter, no descent can be bounded safely, so objects and arrays are copied without the call stack right
+away. Copying keeps working exactly as it does otherwise - the same values come out, and deeply nested values are copied
+just as safely - but copying is slower, because the containers no longer copy themselves. Copying the benchmark
+documents takes 9% (`canada.json`) to 34% (`twitter.json`) longer; values built mostly from objects are affected the
+most.
+
+## Default definition
+
+By default, `#!cpp JSON_NO_THREAD_LOCAL` is not defined.
+
+```cpp
+#undef JSON_NO_THREAD_LOCAL
+```
+
+The library defines it by itself for Clang targeting MinGW, which does not survive the `#!cpp thread_local` storage:
+copying a value segfaults there, with both old and current Clang versions, while GCC targeting MinGW is unaffected.
+
+## Examples
+
+??? example
+
+    The code below forces the library not to use `#!cpp thread_local` storage.
+
+    ```cpp
+    #define JSON_NO_THREAD_LOCAL 1
+    #include <nlohmann/json.hpp>
+
+    ...
+    ```
+
+## Version history
+
+- Added in version 3.12.1.

docs/mkdocs/docs/features/macros.md

@@ -91,6 +91,13 @@ security reasons (e.g., Intel Software Guard Extensions (SGX)).
 
 See [full documentation of `JSON_NO_IO`](../api/macros/json_no_io.md).
 
+## `JSON_NO_THREAD_LOCAL`
+
+When defined, the library does not use `#!cpp thread_local` storage. Copying a value then always avoids the call stack
+rather than descending into a bounded number of levels first, which is slower but yields the same values.
+
+See [full documentation of `JSON_NO_THREAD_LOCAL`](../api/macros/json_no_thread_local.md).
+
 ## `JSON_SKIP_LIBRARY_VERSION_CHECK`
 
 When defined, the library will not create a compiler warning when a different version of the library was already

docs/mkdocs/mkdocs.yml

@@ -291,6 +291,7 @@ nav:
           - 'JSON_HAS_THREE_WAY_COMPARISON': api/macros/json_has_three_way_comparison.md
           - 'JSON_NOEXCEPTION': api/macros/json_noexception.md
           - 'JSON_NO_IO': api/macros/json_no_io.md
+          - 'JSON_NO_THREAD_LOCAL': api/macros/json_no_thread_local.md
           - 'JSON_SKIP_LIBRARY_VERSION_CHECK': api/macros/json_skip_library_version_check.md
           - 'JSON_SKIP_UNSUPPORTED_COMPILER_CHECK': api/macros/json_skip_unsupported_compiler_check.md
           - 'JSON_USE_GLOBAL_UDLS': api/macros/json_use_global_udls.md

include/nlohmann/detail/macro_scope.hpp

@@ -186,6 +186,15 @@
     #define JSON_NO_UNIQUE_ADDRESS
 #endif
 
+// Clang targeting MinGW does not survive the thread_local storage the copy
+// constructor uses to bound its descent: every test that copies a value
+// segfaults with clang 11.0.1 and clang 18.1.8, while the same tests pass with
+// GCC targeting MinGW and with every other toolchain the library is tested on.
+// Copying works the same way without the counter, only more slowly.
+#if !defined(JSON_NO_THREAD_LOCAL) && defined(__clang__) && defined(__MINGW32__)
+    #define JSON_NO_THREAD_LOCAL 1
+#endif
+
 // disable documentation warnings on clang
 #if defined(__clang__)
     #pragma clang diagnostic push

include/nlohmann/json.hpp

@@ -28,14 +28,14 @@
     #pragma GCC diagnostic ignored "-Wignored-attributes"
 #endif
 
-#include <algorithm> // all_of, find, for_each
+#include <algorithm> // all_of, find, for_each, none_of
 #include <cstddef> // nullptr_t, ptrdiff_t, size_t
 #include <functional> // hash, less
 #include <initializer_list> // initializer_list
 #ifndef JSON_NO_IO
     #include <iosfwd> // istream, ostream
 #endif  // JSON_NO_IO
-#include <iterator> // random_access_iterator_tag
+#include <iterator> // make_move_iterator, random_access_iterator_tag
 #include <memory> // unique_ptr
 #include <string> // string, stoi, to_string
 #include <utility> // declval, forward, move, pair, swap
@@ -821,6 +821,336 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return j;
     }
 
+    /// the number of levels an operation descends into before it finishes the
+    /// value below it without the call stack
+    static constexpr std::size_t nesting_depth_limit()
+    {
+        return 128;
+    }
+
+#ifndef JSON_NO_THREAD_LOCAL
+    /*!
+    @brief how many levels the operation going on in this thread has descended into
+
+    Copying a value and comparing two values share this count. The library never
+    nests one inside the other - copying a value does not compare one, and
+    comparing two values does not copy them - and where user code nests them
+    anyway, sharing the count only ends a descent sooner than it had to, which
+    costs a little speed and is never wrong.
+
+    A byte is enough: the count never exceeds the limit by more than the single
+    level that notices the limit has been reached.
+    */
+    static std::size_t& nesting_depth() noexcept
+    {
+        static thread_local std::size_t depth = 0; // NOLINT(misc-use-internal-linkage)
+        return depth;
+    }
+#endif
+
+    /*!
+    @brief counts one level of a bounded descent for as long as it runs
+
+    The count is taken rather than looked up here, because the caller has looked
+    it up already to test it against the limit: reaching thread-local storage is
+    not free, and the path that is taken almost every time should reach it once
+    rather than twice.
+    */
+    class nesting_depth_guard
+    {
+      public:
+        explicit nesting_depth_guard(std::size_t& depth) noexcept
+            : m_depth(depth)
+        {
+            ++m_depth;
+        }
+
+        ~nesting_depth_guard()
+        {
+            --m_depth;
+        }
+
+        nesting_depth_guard(const nesting_depth_guard&) = delete;
+        nesting_depth_guard& operator=(const nesting_depth_guard&) = delete;
+        nesting_depth_guard(nesting_depth_guard&&) = delete;
+        nesting_depth_guard& operator=(nesting_depth_guard&&) = delete;
+
+      private:
+        std::size_t& m_depth;
+    };
+
+    /// an entry of the iterative deep copy's worklist: a structured value and
+    /// the value that is to become its copy
+    using copy_worklist_t = std::vector<std::pair<const basic_json*, basic_json*>>;
+
+    /// scratch space to build the key skeleton of an object copy in one go
+    using copy_scratch_t = std::vector<std::pair<typename object_t::key_type, basic_json>>;
+
+    /// @brief copy everything of @a src into @a dst but its type and value
+    static void copy_metadata(const basic_json& src, basic_json& dst)
+    {
+        // a custom base class is only required to be copy-constructible and
+        // move-assignable, so the copy has to go through a temporary
+        static_cast<json_base_class_t&>(dst) = json_base_class_t(static_cast<const json_base_class_t&>(src));
+
+#if JSON_DIAGNOSTIC_POSITIONS
+        dst.start_position = src.start_position;
+        dst.end_position = src.end_position;
+#else
+        static_cast<void>(src);
+        static_cast<void>(dst);
+#endif
+    }
+
+    /*!
+    @brief copy the value of @a src into @a dst, which must not be structured
+
+    Objects and arrays are left alone: creating those is the one thing the copy
+    constructor and @ref copy_shallow do differently from one another, and it is
+    the reason copying a value can descend at all.
+    */
+    /// @note inlined on purpose: both callers have already told an object or an
+    ///       array apart from the rest, and letting the compiler fold that test
+    ///       into this switch is worth a few percent when copying a value made
+    ///       mostly of numbers
+    JSON_HEDLEY_ALWAYS_INLINE
+    static void copy_leaf_value(const basic_json& src, basic_json& dst)
+    {
+        switch (src.m_data.m_type)
+        {
+            case value_t::string:
+            {
+                dst.m_data.m_value = *src.m_data.m_value.string;
+                break;
+            }
+
+            case value_t::binary:
+            {
+                dst.m_data.m_value = *src.m_data.m_value.binary;
+                break;
+            }
+
+            case value_t::boolean:
+            {
+                dst.m_data.m_value = src.m_data.m_value.boolean;
+                break;
+            }
+
+            case value_t::number_integer:
+            {
+                dst.m_data.m_value = src.m_data.m_value.number_integer;
+                break;
+            }
+
+            case value_t::number_unsigned:
+            {
+                dst.m_data.m_value = src.m_data.m_value.number_unsigned;
+                break;
+            }
+
+            case value_t::number_float:
+            {
+                dst.m_data.m_value = src.m_data.m_value.number_float;
+                break;
+            }
+
+            case value_t::object:
+            case value_t::array:
+            case value_t::null:
+            case value_t::discarded:
+            default:
+                break;
+        }
+    }
+
+    /*!
+    @brief copy everything of @a src into the null value @a dst but the children
+
+    Objects and arrays are not copied here; they are appended to @a worklist to
+    be created later by @ref copy_iteratively. Until that happens, @a dst remains
+    a null value, so that a partially built copy can be destroyed at any point
+    without ever violating the class invariants.
+    */
+    static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
+    {
+        copy_metadata(src, dst);
+
+        if (src.m_data.m_type == value_t::object || src.m_data.m_type == value_t::array)
+        {
+            // defer: dst stays a null value until its container exists
+            worklist.emplace_back(&src, &dst);
+            return;
+        }
+
+        copy_leaf_value(src, dst);
+
+        // only now that the value exists may the type be set: had the creation
+        // of the value thrown, dst would have been left as a valid null value
+        dst.m_data.m_type = src.m_data.m_type;
+    }
+
+    /// @brief create the copy of the array @a src in @a dst
+    /// @note structured elements are appended to @a worklist instead
+    static void copy_array_level(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
+    {
+        const array_t& src_array = *src.m_data.m_value.array;
+
+        // create all elements up front: growing the array afterwards could
+        // invalidate the pointers that are handed to the worklist
+        dst.m_data.m_value.array = create<array_t>(src_array.size(), basic_json());
+
+        auto dst_it = dst.m_data.m_value.array->begin();
+        for (auto src_it = src_array.cbegin(); src_it != src_array.cend(); ++src_it, ++dst_it)
+        {
+            copy_shallow(*src_it, *dst_it, worklist);
+        }
+    }
+
+    /// @brief create the copy of the object @a src in @a dst
+    /// @note structured values are appended to @a worklist instead
+    static void copy_object_level(const basic_json& src, basic_json& dst,
+                                  copy_worklist_t& worklist, copy_scratch_t& scratch)
+    {
+        const object_t& src_object = *src.m_data.m_value.object;
+
+        // build the complete key skeleton and hand it to the object's range
+        // constructor: adding the keys one by one would be quadratic for object
+        // types that are backed by a vector, such as nlohmann::ordered_map
+        scratch.clear();
+        scratch.reserve(src_object.size());
+        for (const auto& element : src_object)
+        {
+            scratch.emplace_back(element.first, basic_json());
+        }
+
+        dst.m_data.m_value.object = create<object_t>(std::make_move_iterator(scratch.begin()),
+                                    std::make_move_iterator(scratch.end()));
+        scratch.clear();
+
+        // pair every value of the copy with its counterpart in the original;
+        // both are enumerated in the same order for every object type with a
+        // deterministic order, so the lookup is only needed for exotic ones
+        auto src_it = src_object.cbegin();
+        for (auto& element : *dst.m_data.m_value.object)
+        {
+            if (JSON_HEDLEY_LIKELY(src_it != src_object.cend() && src_it->first == element.first))
+            {
+                copy_shallow(src_it->second, element.second, worklist);
+                ++src_it;
+            }
+            else
+            {
+                const auto found = src_object.find(element.first);
+                JSON_ASSERT(found != src_object.cend());
+                copy_shallow(found->second, element.second, worklist);
+            }
+        }
+    }
+
+    /*!
+    @brief deep-copy the object or array @a src into this value without recursing
+
+    The values whose copy has not been created yet are kept on an explicit
+    worklist rather than on the call stack. This is only reached for values
+    nested deeper than @ref nesting_depth_limit levels, which is why it copies
+    every container by hand instead of letting the container do it: the fast
+    ways of doing so would descend into the elements and defeat the purpose.
+    */
+    void copy_iteratively(const basic_json& src)
+    {
+        copy_worklist_t worklist;
+        copy_scratch_t scratch;
+
+        const basic_json* src_value = &src;
+        basic_json* dst_value = this;
+
+        for (;;)
+        {
+            if (src_value->m_data.m_type == value_t::array)
+            {
+                copy_array_level(*src_value, *dst_value, worklist);
+            }
+            else
+            {
+                copy_object_level(*src_value, *dst_value, worklist, scratch);
+            }
+
+            // the container is complete and will not be modified again
+            dst_value->set_parents();
+
+            if (worklist.empty())
+            {
+                break;
+            }
+
+            const auto& next = worklist.back();
+            src_value = next.first;
+            dst_value = next.second;
+            worklist.pop_back();
+
+            // the value stops being a null value exactly here
+            dst_value->m_data.m_type = src_value->m_data.m_type;
+        }
+    }
+
+    /*!
+    @brief copy one level of the object or array @a src into this value
+
+    The container copies its own elements, which is the fastest way to fill it.
+    Every element that is structured itself comes back to @ref copy_structured.
+    */
+    void copy_level(const basic_json& src)
+    {
+        if (m_data.m_type == value_t::object)
+        {
+            m_data.m_value = *src.m_data.m_value.object;
+        }
+        else
+        {
+            m_data.m_value = *src.m_data.m_value.array;
+        }
+
+        set_parents();
+    }
+
+    /*!
+    @brief deep-copy the object or array @a src into this value
+
+    Copying a container copies its elements, so a value nested deeply enough
+    used to exhaust the call stack. The descent is bounded here: the first
+    @ref nesting_depth_limit levels are copied by the containers themselves, just
+    as they always were, and anything below that is copied without the call
+    stack by @ref copy_iteratively. Copying a value can therefore no longer
+    exhaust the stack, however deeply it is nested, just like destroying one
+    cannot since #1436.
+
+    Nothing has to be scanned or built by hand to reach that: a value that is
+    not nested deeper than the limit - all but a vanishing minority - is copied
+    exactly as it was before, and this whole detour costs it one counter.
+
+    @sa https://github.com/nlohmann/json/issues/5387
+    */
+    void copy_structured(const basic_json& src)
+    {
+#ifndef JSON_NO_THREAD_LOCAL
+        std::size_t& depth = nesting_depth();
+
+        if (JSON_HEDLEY_LIKELY(depth < nesting_depth_limit()))
+        {
+            const nesting_depth_guard guard(depth);
+            copy_level(src);
+            return;
+        }
+#endif
+
+        // Finish this value without descending any further. It is completed
+        // before this returns, so a copy made by a custom base class - or by
+        // anything else that runs while a copy is going on - is unaffected by
+        // the copy it is nested in.
+        copy_iteratively(src);
+    }
+
+
   public:
     //////////////////////////
     // JSON parser callback //
@@ -1200,60 +1530,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // check of passed value is valid
         other.assert_invariant();
 
-        switch (m_data.m_type)
+        if (m_data.m_type == value_t::object || m_data.m_type == value_t::array)
         {
-            case value_t::object:
+            // copying the container directly would call this constructor again
-            {
+            // for every element, once per nesting level
-                m_data.m_value = *other.m_data.m_value.object;
+            copy_structured(other);
-                break;
+        }
-            }
+        else
-
+        {
-            case value_t::array:
+            copy_leaf_value(other, *this);
-            {
-                m_data.m_value = *other.m_data.m_value.array;
-                break;
-            }
-
-            case value_t::string:
-            {
-                m_data.m_value = *other.m_data.m_value.string;
-                break;
-            }
-
-            case value_t::boolean:
-            {
-                m_data.m_value = other.m_data.m_value.boolean;
-                break;
-            }
-
-            case value_t::number_integer:
-            {
-                m_data.m_value = other.m_data.m_value.number_integer;
-                break;
-            }
-
-            case value_t::number_unsigned:
-            {
-                m_data.m_value = other.m_data.m_value.number_unsigned;
-                break;
-            }
-
-            case value_t::number_float:
-            {
-                m_data.m_value = other.m_data.m_value.number_float;
-                break;
-            }
-
-            case value_t::binary:
-            {
-                m_data.m_value = *other.m_data.m_value.binary;
-                break;
-            }
-
-            case value_t::null:
-            case value_t::discarded:
-            default:
-                break;
         }
 
         set_parents();

single_include/nlohmann/json.hpp

@@ -28,14 +28,14 @@
     #pragma GCC diagnostic ignored "-Wignored-attributes"
 #endif
 
-#include <algorithm> // all_of, find, for_each
+#include <algorithm> // all_of, find, for_each, none_of
 #include <cstddef> // nullptr_t, ptrdiff_t, size_t
 #include <functional> // hash, less
 #include <initializer_list> // initializer_list
 #ifndef JSON_NO_IO
     #include <iosfwd> // istream, ostream
 #endif  // JSON_NO_IO
-#include <iterator> // random_access_iterator_tag
+#include <iterator> // make_move_iterator, random_access_iterator_tag
 #include <memory> // unique_ptr
 #include <string> // string, stoi, to_string
 #include <utility> // declval, forward, move, pair, swap
@@ -2561,6 +2561,15 @@ JSON_HEDLEY_DIAGNOSTIC_POP
     #define JSON_NO_UNIQUE_ADDRESS
 #endif
 
+// Clang targeting MinGW does not survive the thread_local storage the copy
+// constructor uses to bound its descent: every test that copies a value
+// segfaults with clang 11.0.1 and clang 18.1.8, while the same tests pass with
+// GCC targeting MinGW and with every other toolchain the library is tested on.
+// Copying works the same way without the counter, only more slowly.
+#if !defined(JSON_NO_THREAD_LOCAL) && defined(__clang__) && defined(__MINGW32__)
+    #define JSON_NO_THREAD_LOCAL 1
+#endif
+
 // disable documentation warnings on clang
 #if defined(__clang__)
     #pragma clang diagnostic push
@@ -22166,6 +22175,336 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return j;
     }
 
+    /// the number of levels an operation descends into before it finishes the
+    /// value below it without the call stack
+    static constexpr std::size_t nesting_depth_limit()
+    {
+        return 128;
+    }
+
+#ifndef JSON_NO_THREAD_LOCAL
+    /*!
+    @brief how many levels the operation going on in this thread has descended into
+
+    Copying a value and comparing two values share this count. The library never
+    nests one inside the other - copying a value does not compare one, and
+    comparing two values does not copy them - and where user code nests them
+    anyway, sharing the count only ends a descent sooner than it had to, which
+    costs a little speed and is never wrong.
+
+    A byte is enough: the count never exceeds the limit by more than the single
+    level that notices the limit has been reached.
+    */
+    static std::size_t& nesting_depth() noexcept
+    {
+        static thread_local std::size_t depth = 0; // NOLINT(misc-use-internal-linkage)
+        return depth;
+    }
+#endif
+
+    /*!
+    @brief counts one level of a bounded descent for as long as it runs
+
+    The count is taken rather than looked up here, because the caller has looked
+    it up already to test it against the limit: reaching thread-local storage is
+    not free, and the path that is taken almost every time should reach it once
+    rather than twice.
+    */
+    class nesting_depth_guard
+    {
+      public:
+        explicit nesting_depth_guard(std::size_t& depth) noexcept
+            : m_depth(depth)
+        {
+            ++m_depth;
+        }
+
+        ~nesting_depth_guard()
+        {
+            --m_depth;
+        }
+
+        nesting_depth_guard(const nesting_depth_guard&) = delete;
+        nesting_depth_guard& operator=(const nesting_depth_guard&) = delete;
+        nesting_depth_guard(nesting_depth_guard&&) = delete;
+        nesting_depth_guard& operator=(nesting_depth_guard&&) = delete;
+
+      private:
+        std::size_t& m_depth;
+    };
+
+    /// an entry of the iterative deep copy's worklist: a structured value and
+    /// the value that is to become its copy
+    using copy_worklist_t = std::vector<std::pair<const basic_json*, basic_json*>>;
+
+    /// scratch space to build the key skeleton of an object copy in one go
+    using copy_scratch_t = std::vector<std::pair<typename object_t::key_type, basic_json>>;
+
+    /// @brief copy everything of @a src into @a dst but its type and value
+    static void copy_metadata(const basic_json& src, basic_json& dst)
+    {
+        // a custom base class is only required to be copy-constructible and
+        // move-assignable, so the copy has to go through a temporary
+        static_cast<json_base_class_t&>(dst) = json_base_class_t(static_cast<const json_base_class_t&>(src));
+
+#if JSON_DIAGNOSTIC_POSITIONS
+        dst.start_position = src.start_position;
+        dst.end_position = src.end_position;
+#else
+        static_cast<void>(src);
+        static_cast<void>(dst);
+#endif
+    }
+
+    /*!
+    @brief copy the value of @a src into @a dst, which must not be structured
+
+    Objects and arrays are left alone: creating those is the one thing the copy
+    constructor and @ref copy_shallow do differently from one another, and it is
+    the reason copying a value can descend at all.
+    */
+    /// @note inlined on purpose: both callers have already told an object or an
+    ///       array apart from the rest, and letting the compiler fold that test
+    ///       into this switch is worth a few percent when copying a value made
+    ///       mostly of numbers
+    JSON_HEDLEY_ALWAYS_INLINE
+    static void copy_leaf_value(const basic_json& src, basic_json& dst)
+    {
+        switch (src.m_data.m_type)
+        {
+            case value_t::string:
+            {
+                dst.m_data.m_value = *src.m_data.m_value.string;
+                break;
+            }
+
+            case value_t::binary:
+            {
+                dst.m_data.m_value = *src.m_data.m_value.binary;
+                break;
+            }
+
+            case value_t::boolean:
+            {
+                dst.m_data.m_value = src.m_data.m_value.boolean;
+                break;
+            }
+
+            case value_t::number_integer:
+            {
+                dst.m_data.m_value = src.m_data.m_value.number_integer;
+                break;
+            }
+
+            case value_t::number_unsigned:
+            {
+                dst.m_data.m_value = src.m_data.m_value.number_unsigned;
+                break;
+            }
+
+            case value_t::number_float:
+            {
+                dst.m_data.m_value = src.m_data.m_value.number_float;
+                break;
+            }
+
+            case value_t::object:
+            case value_t::array:
+            case value_t::null:
+            case value_t::discarded:
+            default:
+                break;
+        }
+    }
+
+    /*!
+    @brief copy everything of @a src into the null value @a dst but the children
+
+    Objects and arrays are not copied here; they are appended to @a worklist to
+    be created later by @ref copy_iteratively. Until that happens, @a dst remains
+    a null value, so that a partially built copy can be destroyed at any point
+    without ever violating the class invariants.
+    */
+    static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
+    {
+        copy_metadata(src, dst);
+
+        if (src.m_data.m_type == value_t::object || src.m_data.m_type == value_t::array)
+        {
+            // defer: dst stays a null value until its container exists
+            worklist.emplace_back(&src, &dst);
+            return;
+        }
+
+        copy_leaf_value(src, dst);
+
+        // only now that the value exists may the type be set: had the creation
+        // of the value thrown, dst would have been left as a valid null value
+        dst.m_data.m_type = src.m_data.m_type;
+    }
+
+    /// @brief create the copy of the array @a src in @a dst
+    /// @note structured elements are appended to @a worklist instead
+    static void copy_array_level(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
+    {
+        const array_t& src_array = *src.m_data.m_value.array;
+
+        // create all elements up front: growing the array afterwards could
+        // invalidate the pointers that are handed to the worklist
+        dst.m_data.m_value.array = create<array_t>(src_array.size(), basic_json());
+
+        auto dst_it = dst.m_data.m_value.array->begin();
+        for (auto src_it = src_array.cbegin(); src_it != src_array.cend(); ++src_it, ++dst_it)
+        {
+            copy_shallow(*src_it, *dst_it, worklist);
+        }
+    }
+
+    /// @brief create the copy of the object @a src in @a dst
+    /// @note structured values are appended to @a worklist instead
+    static void copy_object_level(const basic_json& src, basic_json& dst,
+                                  copy_worklist_t& worklist, copy_scratch_t& scratch)
+    {
+        const object_t& src_object = *src.m_data.m_value.object;
+
+        // build the complete key skeleton and hand it to the object's range
+        // constructor: adding the keys one by one would be quadratic for object
+        // types that are backed by a vector, such as nlohmann::ordered_map
+        scratch.clear();
+        scratch.reserve(src_object.size());
+        for (const auto& element : src_object)
+        {
+            scratch.emplace_back(element.first, basic_json());
+        }
+
+        dst.m_data.m_value.object = create<object_t>(std::make_move_iterator(scratch.begin()),
+                                    std::make_move_iterator(scratch.end()));
+        scratch.clear();
+
+        // pair every value of the copy with its counterpart in the original;
+        // both are enumerated in the same order for every object type with a
+        // deterministic order, so the lookup is only needed for exotic ones
+        auto src_it = src_object.cbegin();
+        for (auto& element : *dst.m_data.m_value.object)
+        {
+            if (JSON_HEDLEY_LIKELY(src_it != src_object.cend() && src_it->first == element.first))
+            {
+                copy_shallow(src_it->second, element.second, worklist);
+                ++src_it;
+            }
+            else
+            {
+                const auto found = src_object.find(element.first);
+                JSON_ASSERT(found != src_object.cend());
+                copy_shallow(found->second, element.second, worklist);
+            }
+        }
+    }
+
+    /*!
+    @brief deep-copy the object or array @a src into this value without recursing
+
+    The values whose copy has not been created yet are kept on an explicit
+    worklist rather than on the call stack. This is only reached for values
+    nested deeper than @ref nesting_depth_limit levels, which is why it copies
+    every container by hand instead of letting the container do it: the fast
+    ways of doing so would descend into the elements and defeat the purpose.
+    */
+    void copy_iteratively(const basic_json& src)
+    {
+        copy_worklist_t worklist;
+        copy_scratch_t scratch;
+
+        const basic_json* src_value = &src;
+        basic_json* dst_value = this;
+
+        for (;;)
+        {
+            if (src_value->m_data.m_type == value_t::array)
+            {
+                copy_array_level(*src_value, *dst_value, worklist);
+            }
+            else
+            {
+                copy_object_level(*src_value, *dst_value, worklist, scratch);
+            }
+
+            // the container is complete and will not be modified again
+            dst_value->set_parents();
+
+            if (worklist.empty())
+            {
+                break;
+            }
+
+            const auto& next = worklist.back();
+            src_value = next.first;
+            dst_value = next.second;
+            worklist.pop_back();
+
+            // the value stops being a null value exactly here
+            dst_value->m_data.m_type = src_value->m_data.m_type;
+        }
+    }
+
+    /*!
+    @brief copy one level of the object or array @a src into this value
+
+    The container copies its own elements, which is the fastest way to fill it.
+    Every element that is structured itself comes back to @ref copy_structured.
+    */
+    void copy_level(const basic_json& src)
+    {
+        if (m_data.m_type == value_t::object)
+        {
+            m_data.m_value = *src.m_data.m_value.object;
+        }
+        else
+        {
+            m_data.m_value = *src.m_data.m_value.array;
+        }
+
+        set_parents();
+    }
+
+    /*!
+    @brief deep-copy the object or array @a src into this value
+
+    Copying a container copies its elements, so a value nested deeply enough
+    used to exhaust the call stack. The descent is bounded here: the first
+    @ref nesting_depth_limit levels are copied by the containers themselves, just
+    as they always were, and anything below that is copied without the call
+    stack by @ref copy_iteratively. Copying a value can therefore no longer
+    exhaust the stack, however deeply it is nested, just like destroying one
+    cannot since #1436.
+
+    Nothing has to be scanned or built by hand to reach that: a value that is
+    not nested deeper than the limit - all but a vanishing minority - is copied
+    exactly as it was before, and this whole detour costs it one counter.
+
+    @sa https://github.com/nlohmann/json/issues/5387
+    */
+    void copy_structured(const basic_json& src)
+    {
+#ifndef JSON_NO_THREAD_LOCAL
+        std::size_t& depth = nesting_depth();
+
+        if (JSON_HEDLEY_LIKELY(depth < nesting_depth_limit()))
+        {
+            const nesting_depth_guard guard(depth);
+            copy_level(src);
+            return;
+        }
+#endif
+
+        // Finish this value without descending any further. It is completed
+        // before this returns, so a copy made by a custom base class - or by
+        // anything else that runs while a copy is going on - is unaffected by
+        // the copy it is nested in.
+        copy_iteratively(src);
+    }
+
+
   public:
     //////////////////////////
     // JSON parser callback //
@@ -22545,60 +22884,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // check of passed value is valid
         other.assert_invariant();
 
-        switch (m_data.m_type)
+        if (m_data.m_type == value_t::object || m_data.m_type == value_t::array)
         {
-            case value_t::object:
+            // copying the container directly would call this constructor again
-            {
+            // for every element, once per nesting level
-                m_data.m_value = *other.m_data.m_value.object;
+            copy_structured(other);
-                break;
+        }
-            }
+        else
-
+        {
-            case value_t::array:
+            copy_leaf_value(other, *this);
-            {
-                m_data.m_value = *other.m_data.m_value.array;
-                break;
-            }
-
-            case value_t::string:
-            {
-                m_data.m_value = *other.m_data.m_value.string;
-                break;
-            }
-
-            case value_t::boolean:
-            {
-                m_data.m_value = other.m_data.m_value.boolean;
-                break;
-            }
-
-            case value_t::number_integer:
-            {
-                m_data.m_value = other.m_data.m_value.number_integer;
-                break;
-            }
-
-            case value_t::number_unsigned:
-            {
-                m_data.m_value = other.m_data.m_value.number_unsigned;
-                break;
-            }
-
-            case value_t::number_float:
-            {
-                m_data.m_value = other.m_data.m_value.number_float;
-                break;
-            }
-
-            case value_t::binary:
-            {
-                m_data.m_value = *other.m_data.m_value.binary;
-                break;
-            }
-
-            case value_t::null:
-            case value_t::discarded:
-            default:
-                break;
         }
 
         set_parents();

tests/src/unit-allocator.cpp

@@ -216,6 +216,57 @@ TEST_CASE("controlled bad_alloc")
             CHECK_THROWS_AS(my_json(s), std::bad_alloc&);
             next_construct_fails = false;
         }
+
+        SECTION("basic_json(const basic_json&) of a deeply nested value (#5387)")
+        {
+            // Copying a value nested deeper than the descent bound builds the
+            // copy from the top down: every value whose own copy has not been
+            // made yet stays a null value until it is. Failing an allocation
+            // part-way through is what proves such a half-built copy can still
+            // be destroyed.
+            //
+            // Which path the failure lands in depends on the build: the first
+            // allocation of a copy belongs to the outermost level, so here it
+            // is the descending one. Built with JSON_NO_THREAD_LOCAL - as the
+            // ci_test_no_thread_local target builds the whole suite - no
+            // descent is made at all and the very same failure lands in the
+            // iterative path instead, part-way through its worklist.
+            const auto check_deep_copy = [](bool objects)
+            {
+                CAPTURE(objects);
+
+                next_construct_fails = false;
+
+                // deeper than the 128 levels the copy constructor descends into
+                const std::size_t depth = 300;
+
+                my_json j = 1;
+                for (std::size_t i = 0; i < depth; ++i)
+                {
+                    if (objects)
+                    {
+                        my_json wrapper = my_json::object();
+                        wrapper["a"] = std::move(j);
+                        j = std::move(wrapper);
+                    }
+                    else
+                    {
+                        j = my_json::array({std::move(j)});
+                    }
+                }
+
+                // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): the copy is what is tested
+                CHECK_NOTHROW(my_json(j));
+
+                next_construct_fails = true;
+                // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): the copy is what is tested
+                CHECK_THROWS_AS(my_json(j), std::bad_alloc&);
+                next_construct_fails = false;
+            };
+
+            check_deep_copy(false);
+            check_deep_copy(true);
+        }
     }
 }
 

tests/src/unit-diagnostic-positions.cpp

@@ -68,6 +68,72 @@ TEST_CASE("Better diagnostics with positions")
         CHECK(j.end_pos() == root.size());
     }
 
+    SECTION("copying keeps the positions of nested values (#5387)")
+    {
+        // Values nested deeper than the copy constructor's descent bound are
+        // copied without the call stack, on a path that has to carry the
+        // positions over itself; shallower ones copy their containers, which
+        // bring the positions along. Both sides of the bound are checked here.
+        const auto check_copy = [](std::size_t depth, bool objects)
+        {
+            CAPTURE(depth)
+            CAPTURE(objects)
+
+            const std::string opening = objects ? R"({"a":)" : "[";
+            const std::string closing = objects ? "}" : "]";
+
+            std::string text;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                text += opening;
+            }
+            text += "12";
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                text += closing;
+            }
+
+            const json original = json::parse(text);
+            const json copy(original); // NOLINT(performance-unnecessary-copy-initialization)
+
+            const json* o = &original;
+            const json* c = &copy;
+            for (std::size_t level = 0; level <= depth; ++level)
+            {
+                CAPTURE(level)
+                REQUIRE(c->start_pos() == o->start_pos());
+                REQUIRE(c->end_pos() == o->end_pos());
+
+                if (level < depth)
+                {
+                    o = objects ? &o->at("a") : &o->at(0);
+                    c = objects ? &c->at("a") : &c->at(0);
+                }
+            }
+        };
+
+        const auto check_arrays = [&check_copy](std::size_t depth)
+        {
+            check_copy(depth, false);
+        };
+        const auto check_objects = [&check_copy](std::size_t depth)
+        {
+            check_copy(depth, true);
+        };
+
+        check_arrays(1);
+        check_arrays(127);
+        check_arrays(128);
+        check_arrays(129);
+        check_arrays(300);
+
+        check_objects(1);
+        check_objects(127);
+        check_objects(128);
+        check_objects(129);
+        check_objects(300);
+    }
+
     SECTION("JSON patch add to primitive parent (#4292)")
     {
         // the JSON Patch "add" target /foo/bar/baz has a string parent

tests/src/unit-diagnostics.cpp

@@ -273,5 +273,62 @@ TEST_CASE("Regression tests for extended diagnostics")
         CHECK(j1["numbers"]["two"] == 2);
         CHECK(j1["string"] == "t");
     }
+
+    SECTION("Regression test for issue #5387 - copying keeps the parents of nested values")
+    {
+        // A value nested deeper than the copy constructor's descent bound is
+        // copied without the call stack. Every container that path creates has
+        // to have the parents of its children set, or the JSON Pointer in the
+        // diagnostic is cut short.
+        const std::size_t depth = 300;
+
+        SECTION("objects")
+        {
+            json j = "not a number";
+            std::string pointer;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                j = json{{"a", j}};
+                pointer += "/a";
+            }
+
+            json const copy(j); // NOLINT(performance-unnecessary-copy-initialization)
+
+            const json* inner = &copy;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                inner = &inner->at("a");
+            }
+
+            std::string const expected = "[json.exception.type_error.302] (" + pointer + ") type must be number, but is string";
+            int i = 0;
+            CHECK_THROWS_WITH_AS(i = inner->get<int>(), expected.c_str(), json::type_error);
+            CHECK(i == 0);
+        }
+
+        SECTION("arrays")
+        {
+            json j = "not a number";
+            std::string pointer;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                j = json::array({j});
+                pointer += "/0";
+            }
+
+            json const copy(j); // NOLINT(performance-unnecessary-copy-initialization)
+
+            const json* inner = &copy;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                inner = &inner->at(0);
+            }
+
+            std::string const expected = "[json.exception.type_error.302] (" + pointer + ") type must be number, but is string";
+            int i = 0;
+            CHECK_THROWS_WITH_AS(i = inner->get<int>(), expected.c_str(), json::type_error);
+            CHECK(i == 0);
+        }
+    }
 }
 

tests/src/unit-large_json.cpp

@@ -12,6 +12,7 @@
 using nlohmann::json;
 
 #include <algorithm>
+#include <string>
 
 TEST_CASE("tests on very large JSONs")
 {
@@ -27,3 +28,153 @@ TEST_CASE("tests on very large JSONs")
     }
 }
 
+namespace
+{
+
+// Descend a chain of single-element containers and return the value at its end,
+// reporting the number of levels traversed in @a depth.
+//
+// The values in the test case below are nested far deeper than the call stack
+// can follow, so they must not be inspected with operator== or dump(): both are
+// still recursive and would overflow the stack themselves.
+const json* innermost_value(const json& j, std::size_t& depth)
+{
+    const json* current = &j;
+    depth = 0;
+
+    while ((current->is_array() || current->is_object()) && !current->empty())
+    {
+        current = current->is_array()
+                  ? &current->front()
+                  : &current->begin().value();
+        ++depth;
+    }
+
+    return current;
+}
+
+} // namespace
+
+TEST_CASE("tests on deeply nested JSONs")
+{
+    // deep enough to exhaust the call stack, but small enough to stay cheap:
+    // parsing is iterative, so building the values below costs little
+    const std::size_t depth = 100000;
+
+    SECTION("issue #5387 - stack overflow in the copy constructor")
+    {
+        SECTION("array")
+        {
+            const json j = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));
+
+            const json copy(j); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested
+
+            std::size_t copy_depth = 0;
+            CHECK(*innermost_value(copy, copy_depth) == 0);
+            CHECK(copy_depth == depth);
+        }
+
+        SECTION("object")
+        {
+            std::string s;
+            s.reserve((6 * depth) + 1);
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                s += "{\"a\":";
+            }
+            s += '1';
+            s.append(depth, '}');
+
+            const json j = json::parse(s);
+
+            const json copy(j); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested
+
+            std::size_t copy_depth = 0;
+            CHECK(*innermost_value(copy, copy_depth) == 1);
+            CHECK(copy_depth == depth);
+        }
+
+        SECTION("copy assignment")
+        {
+            // operator=(basic_json) takes its argument by value, so the deep
+            // copy happens in the copy constructor
+            const json j = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));
+
+            json target;
+            target = j;
+
+            std::size_t target_depth = 0;
+            CHECK(*innermost_value(target, target_depth) == 0);
+            CHECK(target_depth == depth);
+        }
+
+        SECTION("depths around the bound of the recursive descent")
+        {
+            // The copy constructor descends into a bounded number of levels and
+            // completes whatever is below that without the call stack. Cover
+            // every depth around that bound, so that the two ways of copying
+            // are known to meet cleanly - wherever the bound is set.
+            for (std::size_t d = 1; d <= 300; ++d)
+            {
+                CAPTURE(d);
+
+                const json array = json::parse(std::string(d, '[') + '0' + std::string(d, ']'));
+                const json array_copy(array); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested
+                std::size_t array_depth = 0;
+                CHECK(*innermost_value(array_copy, array_depth) == 0);
+                CHECK(array_depth == d);
+
+                std::string object_text;
+                for (std::size_t i = 0; i < d; ++i)
+                {
+                    object_text += "{\"a\":";
+                }
+                object_text += '1';
+                object_text.append(d, '}');
+
+                const json object = json::parse(object_text);
+                const json object_copy(object); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested
+                std::size_t object_depth = 0;
+                CHECK(*innermost_value(object_copy, object_depth) == 1);
+                CHECK(object_depth == d);
+            }
+        }
+
+        SECTION("a value that is deep in one place only")
+        {
+            json j = json::object();
+            j["shallow"] = 1;
+            j["deep"] = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));
+            j["also_shallow"] = json::array({1, 2, 3});
+
+            const json copy(j);
+
+            CHECK(copy["shallow"] == 1);
+            CHECK(copy["also_shallow"] == json::array({1, 2, 3}));
+
+            std::size_t deep_depth = 0;
+            CHECK(*innermost_value(copy["deep"], deep_depth) == 0);
+            CHECK(deep_depth == depth);
+        }
+
+        SECTION("the copy is independent of the original")
+        {
+            const json j = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));
+
+            json copy(j);
+
+            // reach the innermost value without recursing and replace it
+            json* current = &copy;
+            while (current->is_array() && !current->empty())
+            {
+                current = &current->front();
+            }
+            *current = 42;
+
+            std::size_t unused = 0;
+            CHECK(*innermost_value(copy, unused) == 42);
+            CHECK(*innermost_value(j, unused) == 0);
+        }
+    }
+}
+

tests/src/unit-ordered_json.cpp

@@ -81,3 +81,37 @@ TEST_CASE("regression test for issue #3732 - iteration_proxy_value<iter_impl<ord
     };
     static_cast<void>(fn);
 }
+
+TEST_CASE("copying an ordered_json with nested values")
+{
+    // ordered_map is backed by a vector, so copying an object that has
+    // structured values takes a different route than copying a std::map-backed
+    // one; see https://github.com/nlohmann/json/issues/5387
+    ordered_json oj;
+    oj["z"] = 1;
+    oj["a"]["y"] = 2;
+    oj["a"]["b"]["x"] = 3;
+    oj["m"] = {1, 2, {{"w", 4}}};
+
+    const ordered_json copy(oj);
+
+    SECTION("the copy is equal to the original")
+    {
+        CHECK(copy == oj);
+        CHECK(copy.dump() == oj.dump());
+    }
+
+    SECTION("the key order is preserved at every level")
+    {
+        CHECK(copy.dump() == R"({"z":1,"a":{"y":2,"b":{"x":3}},"m":[1,2,{"w":4}]})");
+    }
+
+    SECTION("the copy is independent of the original")
+    {
+        ordered_json mutated(oj);
+        mutated["a"]["b"]["x"] = 99;
+
+        CHECK(oj["a"]["b"]["x"] == 3);
+        CHECK(mutated["a"]["b"]["x"] == 99);
+    }
+}