Fix parse(container, function) for plain functions as callback

Passing a plain function or function pointer as parser callback, as in json::parse(std::string("[1,2]"), cb), stopped compiling in C++11/14/17. The iterator+sentinel overloads added for C++20 ranges are constrained by can_compare_ne, i.e. whether `first != last` is well-formed. The callback's parameter types make basic_json an associated class, so ADL finds the friend operator!=(const_reference, ScalarType): the container converts to basic_json and the function pointer is a scalar. The sentinel overload was thereby selected and failed hard in its body.

can_compare_ne now excludes function pointers and functions as sentinel type, since a callback is never a sentinel. This shared constraint guards parse, accept, sax_parse and all from_* overloads. Checking IteratorType via std::iterator_traits was avoided on purpose: it would instantiate iterator_traits for every two-argument call, which is a hard error on standard libraries with a non-SFINAE-friendly iterator_traits.

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

include/nlohmann/detail/input/input_adapters.hpp

@@ -1312,15 +1312,23 @@ struct can_compare_ne_either_order : std::integral_constant < bool,
     can_compare_ne_impl<IteratorType, SentinelType>::value ||
     can_compare_ne_reversed<IteratorType, SentinelType>::value > {};
 
-// std::nullptr_t is excluded explicitly: a literal `nullptr` passed as a
-// trailing default argument (e.g. parse(s, nullptr, ...)) must never be
-// mistaken for a sentinel, and some compilers (e.g. GCC 4.8) unreliably
-// SFINAE the `operator!=` detection above for std::nullptr_t against
-// container/string types, which would otherwise make such calls ambiguous
-// with the compatible-input overload.
+// Two kinds of SentinelType are excluded explicitly:
+//
+// - std::nullptr_t: a literal `nullptr` passed as a trailing default argument
+//   (e.g. parse(s, nullptr, ...)) must never be mistaken for a sentinel, and
+//   some compilers (e.g. GCC 4.8) unreliably SFINAE the `operator!=` detection
+//   above for std::nullptr_t against container/string types, which would
+//   otherwise make such calls ambiguous with the compatible-input overload.
+// - function pointers and functions: a parser callback passed as plain function
+//   (e.g. parse(s, cb)) is never a sentinel. The callback's parameter types
+//   make basic_json an associated class, so ADL finds the friend
+//   operator!=(const_reference, ScalarType) of basic_json, which makes
+//   `s != cb` well-formed (both convert to basic_json) and would select the
+//   iterator/sentinel overload.
 template<typename IteratorType, typename SentinelType>
 struct can_compare_ne : std::integral_constant < bool,
     !std::is_same<SentinelType, std::nullptr_t>::value &&
+    !std::is_function<typename std::remove_pointer<SentinelType>::type>::value &&
     can_compare_ne_either_order<IteratorType, SentinelType>::value > {};
 
 template<typename T>

include/nlohmann/detail/json_pointer.hpp

@@ -1091,32 +1091,6 @@ class json_pointer
         }
     };
 
-    /*!
-    @brief the number of the deepest step of the previous path that is a prefix of the new one
-
-    Comparing every one of many steps with the beginning of the path would take
-    quadratic time. Instead, the length of the common beginning of the two paths
-    is computed once: a step whose prefix is not longer is a prefix of the new
-    path, too. For few steps, which is the usual case, @ref resolve_plain
-    compares them, which is cheaper.
-    */
-    template<typename Steps>
-    JSON_HEDLEY_NEVER_INLINE static std::size_t deepest_shared_step(const string_t& path, const std::size_t length, const string_t& previous, const Steps& steps) noexcept
-    {
-        const std::size_t same = common_prefix_length(path.data(), previous.data(), (std::min)(length, previous.size()));
-        std::size_t depth = steps.size();
-        for (; depth > 1; --depth)
-        {
-            // a step ends at a token boundary of the previous path
-            const std::size_t prefix = steps[depth - 1].first;
-            if (prefix <= same && (prefix == length || path[prefix] == '/'))
-            {
-                break;
-            }
-        }
-        return depth;
-    }
-
     /*!
     @brief resolve a plain JSON pointer without building its reference tokens
 
@@ -1145,12 +1119,6 @@ class json_pointer
         if (cache.path != nullptr && !steps.empty() && steps[0].second == root)
         {
             const string_t& previous = *cache.path;
-            if (JSON_HEDLEY_UNLIKELY(steps.size() > 16))
-            {
-                depth = deepest_shared_step(path, length, previous, steps);
-            }
-            else
-            {
             for (depth = steps.size(); depth > 1; --depth)
             {
                 // a step ends at a token boundary of the previous path
@@ -1160,7 +1128,6 @@ class json_pointer
                     break;
                 }
             }
-            }
             steps.resize(depth);
         }
         else

single_include/nlohmann/json.hpp

@@ -10421,15 +10421,23 @@ struct can_compare_ne_either_order : std::integral_constant < bool,
     can_compare_ne_impl<IteratorType, SentinelType>::value ||
     can_compare_ne_reversed<IteratorType, SentinelType>::value > {};
 
-// std::nullptr_t is excluded explicitly: a literal `nullptr` passed as a
-// trailing default argument (e.g. parse(s, nullptr, ...)) must never be
-// mistaken for a sentinel, and some compilers (e.g. GCC 4.8) unreliably
-// SFINAE the `operator!=` detection above for std::nullptr_t against
-// container/string types, which would otherwise make such calls ambiguous
-// with the compatible-input overload.
+// Two kinds of SentinelType are excluded explicitly:
+//
+// - std::nullptr_t: a literal `nullptr` passed as a trailing default argument
+//   (e.g. parse(s, nullptr, ...)) must never be mistaken for a sentinel, and
+//   some compilers (e.g. GCC 4.8) unreliably SFINAE the `operator!=` detection
+//   above for std::nullptr_t against container/string types, which would
+//   otherwise make such calls ambiguous with the compatible-input overload.
+// - function pointers and functions: a parser callback passed as plain function
+//   (e.g. parse(s, cb)) is never a sentinel. The callback's parameter types
+//   make basic_json an associated class, so ADL finds the friend
+//   operator!=(const_reference, ScalarType) of basic_json, which makes
+//   `s != cb` well-formed (both convert to basic_json) and would select the
+//   iterator/sentinel overload.
 template<typename IteratorType, typename SentinelType>
 struct can_compare_ne : std::integral_constant < bool,
     !std::is_same<SentinelType, std::nullptr_t>::value &&
+    !std::is_function<typename std::remove_pointer<SentinelType>::type>::value &&
     can_compare_ne_either_order<IteratorType, SentinelType>::value > {};
 
 template<typename T>
@@ -27266,32 +27274,6 @@ class json_pointer
         }
     };
 
-    /*!
-    @brief the number of the deepest step of the previous path that is a prefix of the new one
-
-    Comparing every one of many steps with the beginning of the path would take
-    quadratic time. Instead, the length of the common beginning of the two paths
-    is computed once: a step whose prefix is not longer is a prefix of the new
-    path, too. For few steps, which is the usual case, @ref resolve_plain
-    compares them, which is cheaper.
-    */
-    template<typename Steps>
-    JSON_HEDLEY_NEVER_INLINE static std::size_t deepest_shared_step(const string_t& path, const std::size_t length, const string_t& previous, const Steps& steps) noexcept
-    {
-        const std::size_t same = common_prefix_length(path.data(), previous.data(), (std::min)(length, previous.size()));
-        std::size_t depth = steps.size();
-        for (; depth > 1; --depth)
-        {
-            // a step ends at a token boundary of the previous path
-            const std::size_t prefix = steps[depth - 1].first;
-            if (prefix <= same && (prefix == length || path[prefix] == '/'))
-            {
-                break;
-            }
-        }
-        return depth;
-    }
-
     /*!
     @brief resolve a plain JSON pointer without building its reference tokens
 
@@ -27320,12 +27302,6 @@ class json_pointer
         if (cache.path != nullptr && !steps.empty() && steps[0].second == root)
         {
             const string_t& previous = *cache.path;
-            if (JSON_HEDLEY_UNLIKELY(steps.size() > 16))
-            {
-                depth = deepest_shared_step(path, length, previous, steps);
-            }
-            else
-            {
             for (depth = steps.size(); depth > 1; --depth)
             {
                 // a step ends at a token boundary of the previous path
@@ -27335,7 +27311,6 @@ class json_pointer
                     break;
                 }
             }
-            }
             steps.resize(depth);
         }
         else

tests/src/unit-class_parser.cpp

@@ -3232,3 +3232,40 @@ TEST_CASE("object keys are delivered decoded and validated")
         CHECK(keys_of_format(std::string("\x87\xC3\x28\x91"), json::input_format_t::bon8).empty());
     }
 }
+
+namespace
+{
+// a plain function (not a lambda or std::function) used as parser callback
+bool keep_everything_callback(int /*depth*/, json::parse_event_t /*event*/, json& /*parsed*/)
+{
+    return true;
+}
+} // namespace
+
+TEST_CASE("a plain function is accepted as parser callback for containers")
+{
+    // ADL finds basic_json's friend operator!= for "container != function
+    // pointer", which must not make the iterator/sentinel overloads viable
+
+    const json expected = {1, 2};
+    const std::string text = "[1,2]";
+    const std::vector<std::uint8_t> bytes(text.begin(), text.end());
+    const std::vector<char> chars(text.begin(), text.end());
+
+    CHECK(json::parse(text, keep_everything_callback) == expected);
+    CHECK(json::parse(text, &keep_everything_callback) == expected);
+    CHECK(json::parse(std::string("[1,2]"), keep_everything_callback) == expected);
+    CHECK(json::parse(bytes, keep_everything_callback) == expected);
+    CHECK(json::parse(bytes, &keep_everything_callback) == expected);
+    CHECK(json::parse(chars, keep_everything_callback) == expected);
+    CHECK(json::parse(text.begin(), text.end(), keep_everything_callback) == expected);
+
+    // the sentinel overloads (shared constraint of parse, accept, sax_parse and
+    // the from_* functions) are not viable for a container and a function
+    // pointer, but still are for iterator pairs
+    using callback_pointer = decltype(&keep_everything_callback);
+    CHECK_FALSE((nlohmann::detail::can_compare_ne<std::string, callback_pointer>::value));
+    CHECK_FALSE((nlohmann::detail::can_compare_ne<std::vector<std::uint8_t>, callback_pointer>::value));
+    CHECK((nlohmann::detail::can_compare_ne<std::string::const_iterator, std::string::const_iterator>::value));
+    CHECK((nlohmann::detail::can_compare_ne<const char*, const char*>::value));
+}

tests/src/unit-json_patch.cpp

@@ -14,12 +14,9 @@ using nlohmann::json;
     using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 #endif
 
-#include <algorithm>
 #include <cstddef>
-#include <ctime>
 #include <fstream>
 #include <limits>
-#include <random>
 #include <string>
 #include <utility>
 #include <vector>
@@ -2554,419 +2551,3 @@ TEST_CASE("JSON patch - diff of leaf values below the bound of the recursive des
     CHECK(object_patch.size() == 2);
     CHECK(object_source.patch(object_patch) == object_target);
 }
-
-namespace
-{
-// all pointers to values below `value` (not to `value` itself)
-void collect_pointers(const json& value, const std::string& path, std::vector<std::string>& pointers)
-{
-    if (value.is_object())
-    {
-        for (const auto& element : value.items())
-        {
-            std::string child = path;
-            child += '/';
-            for (const char c : element.key())
-            {
-                if (c == '~')
-                {
-                    child += "~0";
-                }
-                else if (c == '/')
-                {
-                    child += "~1";
-                }
-                else
-                {
-                    child += c;
-                }
-            }
-            pointers.push_back(child);
-            collect_pointers(element.value(), child, pointers);
-        }
-    }
-    else if (value.is_array())
-    {
-        for (std::size_t i = 0; i < value.size(); ++i)
-        {
-            std::string child = path;
-            child += '/';
-            child += std::to_string(i);
-            pointers.push_back(child);
-            collect_pointers(value[i], child, pointers);
-        }
-    }
-}
-
-// a random document of at most `depth` levels, with the names as keys
-json random_document(std::mt19937& gen, const std::vector<std::string>& names, const int depth)
-{
-    const int kind = std::uniform_int_distribution<int>(0, 9)(gen);
-    if (depth == 0 || kind < 2)
-    {
-        return std::uniform_int_distribution<int>(0, 3)(gen);
-    }
-    const int size = std::uniform_int_distribution<int>(1, 3)(gen);
-    json result = kind < 6 ? json::array() : json::object();
-    for (int i = 0; i < size; ++i)
-    {
-        if (kind < 6)
-        {
-            result.push_back(random_document(gen, names, depth - 1));
-        }
-        else
-        {
-            result[names[std::uniform_int_distribution<std::size_t>(0, names.size() - 1)(gen)]] = random_document(gen, names, depth - 1);
-        }
-    }
-    return result;
-}
-
-// the strings, one after the other
-std::string concat(std::string first, const std::string& second, const std::string& third = std::string())
-{
-    first += second;
-    first += third;
-    return first;
-}
-
-// the best of five times in seconds that std::clock() measures to apply the patch
-double patch_cpu_time(json& doc, const json& patch)
-{
-    double best = 1e30;
-    for (int i = 0; i < 5; ++i)
-    {
-        const std::clock_t start = std::clock();
-        doc.patch_inplace(patch);
-        best = (std::min)(best, static_cast<double>(std::clock() - start) / static_cast<double>(CLOCKS_PER_SEC));
-    }
-    return best;
-}
-} // namespace
-
-TEST_CASE("JSON patch - consecutive operations with paths that share a prefix")
-{
-    SECTION("tokens that start the same way")
-    {
-        const json doc = {{"a", {{"bc", {{"x", 1}}}, {"b", {{"x", 2}}}, {"b2", 3}}}};
-
-        // the second path shares "/a/b" with the first, but not the token "bc"
-        const json patch = json::parse(R"([
-            {"op": "test", "path": "/a/bc/x", "value": 1},
-            {"op": "test", "path": "/a/b/x", "value": 2},
-            {"op": "test", "path": "/a/bc/x", "value": 1},
-            {"op": "test", "path": "/a/b2", "value": 3},
-            {"op": "test", "path": "/a/b/x", "value": 2}
-        ])");
-        CHECK(doc.patch(patch) == doc);
-
-        json inplace = doc;
-        inplace.patch_inplace(patch);
-        CHECK(inplace == doc);
-
-        // a path that is a prefix of the previous one, and the other way round
-        const json prefixes = json::parse(R"([
-            {"op": "test", "path": "/a/bc/x", "value": 1},
-            {"op": "test", "path": "/a/bc", "value": {"x": 1}},
-            {"op": "test", "path": "/a/bc/x", "value": 1},
-            {"op": "test", "path": "/a", "value": {"bc": {"x": 1}, "b": {"x": 2}, "b2": 3}},
-            {"op": "test", "path": "/a/b/x", "value": 2},
-            {"op": "test", "path": "", "value": {"a": {"bc": {"x": 1}, "b": {"x": 2}, "b2": 3}}}
-        ])");
-        CHECK(doc.patch(prefixes) == doc);
-    }
-
-    SECTION("operations above a path that was resolved before")
-    {
-        const json doc = json::parse(R"({"a": [{"b": [1, 2, 3]}, {"b": [4, 5, 6]}], "c": {"d": {"e": 7}}})");
-
-        // an element inserted before the elements of an array moves them
-        CHECK(doc.patch(json::parse(R"([
-            {"op": "test", "path": "/a/0/b/2", "value": 3},
-            {"op": "add", "path": "/a/0", "value": {"b": [9]}},
-            {"op": "test", "path": "/a/1/b/2", "value": 3},
-            {"op": "test", "path": "/a/0/b/0", "value": 9},
-            {"op": "test", "path": "/a/2/b/0", "value": 4}
-        ])")) == json::parse(R"({"a": [{"b": [9]}, {"b": [1, 2, 3]}, {"b": [4, 5, 6]}], "c": {"d": {"e": 7}}})"));
-
-        // an element removed from the front
-        CHECK(doc.patch(json::parse(R"([
-            {"op": "test", "path": "/a/1/b/2", "value": 6},
-            {"op": "remove", "path": "/a/0"},
-            {"op": "test", "path": "/a/0/b/2", "value": 6},
-            {"op": "replace", "path": "/a/0/b/2", "value": 60},
-            {"op": "test", "path": "/a/0/b/2", "value": 60}
-        ])")) == json::parse(R"({"a": [{"b": [4, 5, 60]}], "c": {"d": {"e": 7}}})"));
-
-        // a value replaced above the path that was resolved before
-        CHECK(doc.patch(json::parse(R"([
-            {"op": "test", "path": "/c/d/e", "value": 7},
-            {"op": "replace", "path": "/c/d", "value": {"e": 8, "f": {"g": 9}}},
-            {"op": "test", "path": "/c/d/e", "value": 8},
-            {"op": "test", "path": "/c/d/f/g", "value": 9},
-            {"op": "remove", "path": "/c/d/f"},
-            {"op": "add", "path": "/c/d/f", "value": [1]},
-            {"op": "test", "path": "/c/d/f/0", "value": 1}
-        ])")) == json::parse(R"({"a": [{"b": [1, 2, 3]}, {"b": [4, 5, 6]}], "c": {"d": {"e": 8, "f": [1]}}})"));
-
-#if !defined(JSON_NOEXCEPTION)
-        // a value removed above the path that was resolved before is gone
-        const json removed = json::parse(R"([
-            {"op": "test", "path": "/c/d/e", "value": 7},
-            {"op": "remove", "path": "/c/d"},
-            {"op": "test", "path": "/c/d/e", "value": 7}
-        ])");
-        CHECK_THROWS_AS(doc.patch(removed), json::other_error&);
-        json inplace = doc;
-        CHECK_THROWS_AS(inplace.patch_inplace(removed), json::other_error&);
-
-        const json emptied = json::parse(R"([
-            {"op": "test", "path": "/a/1/b/2", "value": 6},
-            {"op": "remove", "path": "/a/1"},
-            {"op": "test", "path": "/a/1/b/2", "value": 6}
-        ])");
-        CHECK_THROWS_AS(doc.patch(emptied), json::other_error&);
-#endif
-    }
-
-    SECTION("paths with more than 16 tokens")
-    {
-        // 40 levels, with an array at the bottom and a second branch at level 20
-        std::string prefix20;
-        std::string prefix40;
-        json doc = {{"leaf", {1, 2, 3}}};
-        for (int i = 40; i > 0; --i)
-        {
-            if (i == 20)
-            {
-                doc["other"] = {{"x", 5}};
-            }
-            doc = {{"n", doc}};
-        }
-        for (int i = 1; i <= 40; ++i)
-        {
-            prefix40 += "/n";
-            if (i == 20)
-            {
-                prefix20 = prefix40;
-            }
-        }
-
-        const auto op = [](const char* name, const std::string & path, const json & value)
-        {
-            return json({{"op", name}, {"path", path}, {"value", value}});
-        };
-        const json patch =
-        {
-            op("test", concat(prefix40, "/leaf/2"), 3),
-            op("test", concat(prefix20, "/other/x"), 5),
-            op("test", concat(prefix40, "/leaf/0"), 1),
-            op("add", concat(prefix40, "/leaf/0"), 0),
-            op("test", concat(prefix40, "/leaf/3"), 3),
-            op("replace", concat(prefix20, "/other"), {{"x", 6}}),
-            op("test", concat(prefix20, "/other/x"), 6),
-            op("test", concat(prefix40, "/leaf/1"), 1),
-            {{"op", "remove"}, {"path", concat(prefix20, "/other")}},
-            op("test", concat(prefix40, "/leaf/1"), 1)
-        };
-        json expected = doc;
-        for (const auto& operation : patch)
-        {
-            expected = expected.patch(json::array({operation}));
-        }
-        CHECK(doc.patch(patch) == expected);
-        json inplace = doc;
-        inplace.patch_inplace(patch);
-        CHECK(inplace == expected);
-        CHECK(expected[json::json_pointer(concat(prefix40, "/leaf"))] == json({0, 1, 2, 3}));
-        CHECK(!expected[json::json_pointer(prefix20)].contains("other"));
-
-#if !defined(JSON_NOEXCEPTION)
-        // the value that was resolved before is gone with the member that held it
-        const json gone =
-        {
-            op("test", concat(prefix40, "/leaf/2"), 3),
-            {{"op", "remove"}, {"path", prefix20}},
-            op("test", concat(prefix40, "/leaf/2"), 3)
-        };
-        CHECK_THROWS_AS(doc.patch(gone), json::other_error&);
-#endif
-    }
-
-    SECTION("random sequences of operations give the same result as one operation at a time")
-    {
-        std::mt19937 gen(20261110); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed): reproducible
-        const std::vector<std::string> names = {"a", "b", "ab", "abc", "a/b", "a~b", "1", "10", "token_longer_than_eight_bytes"};
-        const auto pick = [&gen](const std::size_t n)
-        {
-            return std::uniform_int_distribution<std::size_t>(0, n - 1)(gen);
-        };
-
-        for (int round = 0; round < 300; ++round)
-        {
-            const json start = random_document(gen, names, 7);
-            json current = start;
-            json patch = json::array();
-            std::string previous;
-
-            for (int step = 0; step < 25; ++step)
-            {
-                std::vector<std::string> pointers;
-                collect_pointers(current, "", pointers);
-                if (pointers.empty())
-                {
-                    break;
-                }
-
-                // most of the time, stay below a part of the previous path
-                std::vector<std::string> candidates;
-                if (!previous.empty() && pick(10) < 8)
-                {
-                    std::string prefix = previous;
-                    const std::size_t cut = prefix.find('/', 1 + pick(prefix.size()));
-                    prefix = prefix.substr(0, cut == std::string::npos ? prefix.size() : cut);
-                    for (const auto& p : pointers)
-                    {
-                        if (p.compare(0, prefix.size(), prefix) == 0)
-                        {
-                            candidates.push_back(p);
-                        }
-                    }
-                }
-                if (candidates.empty())
-                {
-                    candidates = pointers;
-                }
-                const std::string path = candidates[pick(candidates.size())];
-                const json::json_pointer pointer(path);
-
-                json operation;
-                switch (pick(5))
-                {
-                    case 0:
-                        operation = {{"op", "test"}, {"path", path}, {"value", current[pointer]}};
-                        break;
-                    case 1:
-                        operation = {{"op", "replace"}, {"path", path}, {"value", static_cast<int>(pick(100))}};
-                        break;
-                    case 2:
-                        operation = {{"op", "remove"}, {"path", path}};
-                        break;
-                    default:
-                    {
-                        // add below an array or object, which may be the root
-                        const json::json_pointer parent_pointer = pick(4) == 0 ? json::json_pointer() : pointer;
-                        const std::string parent_path = parent_pointer.to_string();
-                        const json& parent = current[parent_pointer];
-                        std::string token = "-";
-                        if (parent.is_array())
-                        {
-                            token = pick(3) == 0 ? "-" : std::to_string(pick(parent.size() + 1));
-                        }
-                        else if (parent.is_object())
-                        {
-                            token = pick(2) == 0 ? "new" : "ab";
-                        }
-                        else
-                        {
-                            // a primitive parent: replace it instead
-                            operation = {{"op", "replace"}, {"path", parent_path}, {"value", json::array({1, 2})}};
-                            break;
-                        }
-                        operation = {{"op", "add"}, {"path", concat(parent_path, "/", token)}, {"value", json::object({{"k", {1, 2}}})}};
-                        break;
-                    }
-                }
-                if (operation["path"].get<std::string>().empty())
-                {
-                    continue;
-                }
-
-                // one operation at a time: the cache is empty at the start of each
-                current = current.patch(json::array({operation}));
-                patch.push_back(operation);
-                previous = operation["path"].get<std::string>();
-            }
-
-            CHECK(start.patch(patch) == current);
-            json inplace = start;
-            inplace.patch_inplace(patch);
-            CHECK(inplace == current);
-
-            // the same for a type that keeps the order of the members, which makes
-            // objects that were built in a different order unequal: leave out the
-            // tests of structured values
-            json without_tests = json::array();
-            for (const auto& operation : patch)
-            {
-                if (operation["op"] != "test" || !operation["value"].is_structured())
-                {
-                    without_tests.push_back(operation);
-                }
-            }
-            const nlohmann::ordered_json ordered_patch = without_tests;
-            nlohmann::ordered_json ordered_inplace = start;
-            ordered_inplace.patch_inplace(ordered_patch);
-            CHECK(json(ordered_inplace) == current);
-        }
-    }
-}
-
-TEST_CASE("JSON patch - time is linear for long paths that share a prefix")
-{
-    // The time of the patch is compared with the time of a patch that is
-    // linear for sure, because a bound in seconds would depend on the machine:
-    // both test the same two values at the end of paths with 2n tokens. The
-    // second path of the first patch shares n tokens with the first one and
-    // continues for n tokens: the ratio is 1 to 2, and about 60 when every
-    // step is compared with the start of the path. The time that std::clock()
-    // measures is mostly the time of the processor, which a busy machine does
-    // not inflate (on MSVC, it is the time that has passed).
-    constexpr std::size_t n = 25000;
-
-    const auto nest = [](const std::size_t depth, const std::string & inner)
-    {
-        std::string text;
-        for (std::size_t i = 0; i < depth; ++i)
-        {
-            text += "{\"a\":";
-        }
-        text += inner;
-        text.append(depth, '}');
-        return text;
-    };
-    std::string as;
-    for (std::size_t i = 0; i < n; ++i)
-    {
-        as += "/a";
-    }
-    const std::string deep = nest(n, "1");
-    std::string members = "{\"x\":";
-    members += deep;
-    members += ",\"y\":";
-    members += deep;
-    members += "}";
-    json doc = json::parse(nest(n, members));
-
-    const auto test_op = [](const std::string & path)
-    {
-        return json({{"op", "test"}, {"path", path}, {"value", 1}});
-    };
-    const json shared = json::array({test_op(concat(as, "/x", as)), test_op(concat(as, "/y", as))});
-    const json same = json::array({test_op(concat(as, "/x", as)), test_op(concat(as, "/x", as))});
-
-    // the operations do not change the document, which is too deep to copy;
-    // a busy machine may spoil one comparison, hardly two
-    bool linear = false;
-    double t_shared = 0;
-    double t_same = 0;
-    for (int attempt = 0; attempt < 2 && !linear; ++attempt)
-    {
-        t_shared = patch_cpu_time(doc, shared);
-        t_same = (std::max)(patch_cpu_time(doc, same), 1e-3);
-        linear = t_shared < 10 * t_same;
-    }
-    CAPTURE(t_shared)
-    CAPTURE(t_same)
-    CHECK(linear);
-}