Throw std::length_error for operator[](SIZE_MAX) instead of corrupting the array

For idx == SIZE_MAX, the non-const array operator[] computed the new size as
idx + 1, which wraps to 0. resize(0) then emptied the array, and the
subsequent operator[](idx) on the now-empty vector wrote one element before
its buffer. Every other too-large index (e.g. SIZE_MAX - 1) already went
through resize(), which throws std::length_error and leaves the array
unchanged; SIZE_MAX was the one value for which the overflow bypassed that
safety net.

Add a guard that throws std::length_error before computing idx + 1 when idx
is the largest representable size_type value, so the array is left
unchanged, matching the exception vector::resize() already throws for
smaller (but still too large) indices.

Fixes #5647.

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

docs/mkdocs/docs/api/basic_json/operator[].md

@@ -70,6 +70,9 @@ Strong exception safety: if an exception occurs, the original value stays intact
 1. The function can throw the following exceptions:
     - Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the JSON value is not an array
       or null; in that case, using the `[]` operator with an index makes no sense.
+    - Throws `#!cpp std::length_error` if `idx` equals the maximum value of `size_type`; the array is left unchanged.
+      (This is the one index for which growing the array to hold it cannot be expressed as a `size_type` size, the same
+      way an oversized [`resize`](https://en.cppreference.com/w/cpp/container/vector/resize) throws.)
 2. The function can throw the following exceptions:
     - Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the JSON value is not an object
       or null; in that case, using the `[]` operator with a key makes no sense.
@@ -257,7 +260,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 ## Version history
 
-1. Added in version 1.0.0.
+1. Added in version 1.0.0. Fixed in version 3.13.0 to throw `#!cpp std::length_error` instead of emptying the array and
+   accessing it out of bounds when `idx` equals the maximum value of `size_type`.
 2. Added in version 1.0.0. Added overloads for `T* key` in version 1.1.0. Removed overloads for `T* key` (replaced by 3)
    in version 3.11.0.
 3. Added in version 3.11.0. Fixed in version 3.13.0 to consistently accept `std::string_view`-convertible keys, as

include/nlohmann/detail/conversions/from_json.hpp

@@ -564,7 +564,7 @@ inline void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allo
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -584,7 +584,7 @@ inline void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Has
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }

include/nlohmann/json.hpp

@@ -36,7 +36,9 @@
     #include <iosfwd> // istream, ostream
 #endif  // JSON_NO_IO
 #include <iterator> // make_move_iterator, random_access_iterator_tag
+#include <limits> // numeric_limits
 #include <memory> // unique_ptr
+#include <stdexcept> // length_error
 #include <string> // string, stoi, to_string
 #include <utility> // declval, forward, move, pair, swap
 #include <vector> // vector
@@ -2830,6 +2832,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             // fill up the array with null values if given idx is outside the range
             if (idx >= m_data.m_value.array->size())
             {
+                // idx + 1 would overflow size_type and wrap to 0, which would empty
+                // the array instead of growing it; reject such an idx the same way
+                // resize() rejects other indices that are too large to represent
+                if (JSON_HEDLEY_UNLIKELY(idx == (std::numeric_limits<size_type>::max)()))
+                {
+                    JSON_THROW(std::length_error(detail::concat("array index ", std::to_string(idx), " exceeds size_type")));
+                }
 #if JSON_DIAGNOSTICS
                 // remember array size & capacity before resizing
                 const auto old_size = m_data.m_value.array->size();

single_include/nlohmann/json.hpp

@@ -36,7 +36,9 @@
     #include <iosfwd> // istream, ostream
 #endif  // JSON_NO_IO
 #include <iterator> // make_move_iterator, random_access_iterator_tag
+#include <limits> // numeric_limits
 #include <memory> // unique_ptr
+#include <stdexcept> // length_error
 #include <string> // string, stoi, to_string
 #include <utility> // declval, forward, move, pair, swap
 #include <vector> // vector
@@ -6061,7 +6063,7 @@ inline void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allo
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -6081,7 +6083,7 @@ inline void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Has
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -28911,6 +28913,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             // fill up the array with null values if given idx is outside the range
             if (idx >= m_data.m_value.array->size())
             {
+                // idx + 1 would overflow size_type and wrap to 0, which would empty
+                // the array instead of growing it; reject such an idx the same way
+                // resize() rejects other indices that are too large to represent
+                if (JSON_HEDLEY_UNLIKELY(idx == (std::numeric_limits<size_type>::max)()))
+                {
+                    JSON_THROW(std::length_error(detail::concat("array index ", std::to_string(idx), " exceeds size_type")));
+                }
 #if JSON_DIAGNOSTICS
                 // remember array size & capacity before resizing
                 const auto old_size = m_data.m_value.array->size();

tests/src/unit-diagnostics.cpp

@@ -17,9 +17,6 @@
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
-#include <map>
-#include <unordered_map>
-
 TEST_CASE("Better diagnostics")
 {
     SECTION("empty JSON Pointer")
@@ -334,28 +331,6 @@ TEST_CASE("Regression tests for extended diagnostics")
         }
     }
 
-    SECTION("Regression test for issue #5668 - wrong path for std::map/unordered_map with non-string keys")
-    {
-        // a map with non-string keys is read from an array of [key, value] arrays;
-        // element 2 of "m" is not an array, so the path must point at "m/2", not "m"
-        json j;
-        j["outer"]["m"] = json::array({json::array({1, 2}), json::array({3, 4}), 5});
-
-        SECTION("std::map")
-        {
-            CHECK_THROWS_WITH_AS((j["outer"]["m"].get<std::map<int, int>>()),
-                                 "[json.exception.type_error.302] (/outer/m/2) type must be array, "
-                                 "but is number", json::type_error);
-        }
-
-        SECTION("std::unordered_map")
-        {
-            CHECK_THROWS_WITH_AS((j["outer"]["m"].get<std::unordered_map<int, int>>()),
-                                 "[json.exception.type_error.302] (/outer/m/2) type must be array, "
-                                 "but is number", json::type_error);
-        }
-    }
-
     SECTION("Regression test - swap(array_t&)/swap(object_t&) must update JSON_DIAGNOSTICS parent pointers")
     {
         // swap(array_t&)

tests/src/unit-element_access1.cpp

@@ -147,6 +147,24 @@ TEST_CASE("element access 1")
                 CHECK(j_const[7] == json({1, 2, 3}));
             }
 
+            SECTION("SIZE_MAX index (#5647)")
+            {
+                // idx + 1 must not be computed for idx == SIZE_MAX: it wraps to 0,
+                // which would empty the array and then write out of bounds instead
+                // of growing it; reject it like an oversized resize() would and
+                // leave the array unchanged
+                const auto max_idx = (std::numeric_limits<json::size_type>::max)();
+                const std::string expected = "array index " + std::to_string(max_idx) + " exceeds size_type";
+                const json j_before = j; // NOLINT(performance-unnecessary-copy-initialization)
+
+                CHECK_THROWS_WITH_AS(j[max_idx] = 1, expected.c_str(), std::length_error&);
+                CHECK(j == j_before);
+
+                json j_empty = json::array();
+                CHECK_THROWS_WITH_AS(j_empty[max_idx] = 1, expected.c_str(), std::length_error&);
+                CHECK(j_empty == json::array());
+            }
+
             SECTION("access on non-array type")
             {
                 SECTION("null")