From 165f91ed3c9fb8f3e4a912da7338452575c9c4a1 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Sun, 25 Oct 2026 12:04:43 +0100
Subject: [PATCH] Build the DOM parser with Visual Studio 2015 again

Since #5882, the three VS 2015 jobs of AppVeyor fail on develop:
- the detection of keys that can be indexed instantiated std::hash for
  every string type, which VS 2015 rejects with a static_assert for a type
  it does not hash (alt_string): std::hash is now looked at only for
  objects in a vector
- the frame stack copied its frames with std::copy to a raw pointer, which
  VS 2015 reports as unsafe (C4996, an error with /WX): a loop now

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---
 include/nlohmann/detail/input/json_sax.hpp | 19 +++++++++++++++----
 single_include/nlohmann/json.hpp           | 19 +++++++++++++++----
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/include/nlohmann/detail/input/json_sax.hpp b/include/nlohmann/detail/input/json_sax.hpp
index 6fd0be52d..676b36290 100644
--- a/include/nlohmann/detail/input/json_sax.hpp
+++ b/include/nlohmann/detail/input/json_sax.hpp
@@ -650,9 +650,16 @@ class json_sax_dom_parser : public json_sax_dom_error_state
     /// so that a big object can find the keys it has in a hash table instead of
     /// searching all of them
     template<typename ObjectType, typename StringType>
-    using detect_indexable_keys = decltype(std::declval<ObjectType&>().emplace_new_key(std::declval<const StringType&>(), std::declval<typename ObjectType::mapped_type>()),
-                                           std::hash<StringType>()(std::declval<const StringType&>()));
-    static constexpr bool indexed_keys = is_detected<detect_indexable_keys, object_t, string_t>::value;
+    using detect_emplace_new_key = decltype(std::declval<ObjectType&>().emplace_new_key(std::declval<const StringType&>(), std::declval<typename ObjectType::mapped_type>()));
+    template<typename StringType>
+    using detect_hash = decltype(std::hash<StringType>()(std::declval<const StringType&>()));
+    /// (std::hash is only looked at for objects in a vector: the one of VS 2015
+    /// fails to compile, instead of being undetectable, for a type it does not hash)
+    template<bool InVector, typename StringType>
+    struct has_indexable_keys : std::false_type {};
+    template<typename StringType>
+    struct has_indexable_keys<true, StringType> : is_detected<detect_hash, StringType> {};
+    static constexpr bool indexed_keys = has_indexable_keys<is_detected<detect_emplace_new_key, object_t, string_t>::value, string_t>::value;
 
     /// what a frame holds for objects that are kept in a vector: the key index
     /// of an object with many keys, and for an object that is collected on the
@@ -774,7 +781,11 @@ class json_sax_dom_parser : public json_sax_dom_error_state
         {
             const std::size_t new_capacity = capacity * 2;
             std::unique_ptr<frame[]> new_heap(new frame[new_capacity]); // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-            std::copy(data, data + count, new_heap.get());
+            // (a loop, not std::copy: VS 2015 rejects std::copy to a raw pointer, C4996)
+            for (std::size_t i = 0; i < count; ++i)
+            {
+                new_heap[i] = data[i];
+            }
             heap = std::move(new_heap);
             data = heap.get();
             capacity = new_capacity;
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index a9e40693d..fc4d96ccb 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -15773,9 +15773,16 @@ class json_sax_dom_parser : public json_sax_dom_error_state
     /// so that a big object can find the keys it has in a hash table instead of
     /// searching all of them
     template<typename ObjectType, typename StringType>
-    using detect_indexable_keys = decltype(std::declval<ObjectType&>().emplace_new_key(std::declval<const StringType&>(), std::declval<typename ObjectType::mapped_type>()),
-                                           std::hash<StringType>()(std::declval<const StringType&>()));
-    static constexpr bool indexed_keys = is_detected<detect_indexable_keys, object_t, string_t>::value;
+    using detect_emplace_new_key = decltype(std::declval<ObjectType&>().emplace_new_key(std::declval<const StringType&>(), std::declval<typename ObjectType::mapped_type>()));
+    template<typename StringType>
+    using detect_hash = decltype(std::hash<StringType>()(std::declval<const StringType&>()));
+    /// (std::hash is only looked at for objects in a vector: the one of VS 2015
+    /// fails to compile, instead of being undetectable, for a type it does not hash)
+    template<bool InVector, typename StringType>
+    struct has_indexable_keys : std::false_type {};
+    template<typename StringType>
+    struct has_indexable_keys<true, StringType> : is_detected<detect_hash, StringType> {};
+    static constexpr bool indexed_keys = has_indexable_keys<is_detected<detect_emplace_new_key, object_t, string_t>::value, string_t>::value;
 
     /// what a frame holds for objects that are kept in a vector: the key index
     /// of an object with many keys, and for an object that is collected on the
@@ -15897,7 +15904,11 @@ class json_sax_dom_parser : public json_sax_dom_error_state
         {
             const std::size_t new_capacity = capacity * 2;
             std::unique_ptr<frame[]> new_heap(new frame[new_capacity]); // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-            std::copy(data, data + count, new_heap.get());
+            // (a loop, not std::copy: VS 2015 rejects std::copy to a raw pointer, C4996)
+            for (std::size_t i = 0; i < count; ++i)
+            {
+                new_heap[i] = data[i];
+            }
             heap = std::move(new_heap);
             data = heap.get();
             capacity = new_capacity;