Fixed init-list construction when size_type is not int (#4140)

Author: tomalakgeretkal

include/nlohmann/json.hpp

@@ -906,7 +906,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         bool is_an_object = std::all_of(init.begin(), init.end(),
                                         [](const detail::json_ref<basic_json>& element_ref)
         {
-            return element_ref->is_array() && element_ref->size() == 2 && (*element_ref)[0].is_string();
+            // The cast is to ensure op[size_type] is called, bearing in mind size_type may not be int;
+            // (many string types can be constructed from 0 via its null-pointer guise, so we get a
+            // broken call to op[key_type], the wrong semantics and a 4804 warning on Windows)
+            return element_ref->is_array() && element_ref->size() == 2 && (*element_ref)[static_cast<size_type>(0)].is_string();
         });
 
         // adjust type if type deduction is not wanted

single_include/nlohmann/json.hpp

@@ -20112,7 +20112,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         bool is_an_object = std::all_of(init.begin(), init.end(),
                                         [](const detail::json_ref<basic_json>& element_ref)
         {
-            return element_ref->is_array() && element_ref->size() == 2 && (*element_ref)[0].is_string();
+            // The cast is to ensure op[size_type] is called, bearing in mind size_type may not be int;
+            // (many string types can be constructed from 0 via its null-pointer guise, so we get a
+            // broken call to op[key_type], the wrong semantics and a 4804 warning on Windows)
+            return element_ref->is_array() && element_ref->size() == 2 && (*element_ref)[static_cast<size_type>(0)].is_string();
         });
 
         // adjust type if type deduction is not wanted