diff --git a/.github/external_ci/appveyor.yml b/.github/external_ci/appveyor.yml
index 5c6b47a344..5c4af270ca 100644
--- a/.github/external_ci/appveyor.yml
+++ b/.github/external_ci/appveyor.yml
@@ -48,7 +48,7 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Release
       platform: x86
-      CXX_FLAGS: "/W4 /WX"
+      CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /W4 /WX"
       CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF"
       GENERATOR: Visual Studio 16 2019
 
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index ceb7a9f11d..678761a2c2 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -3499,71 +3499,71 @@ NLOHMANN_JSON_NAMESPACE_END
 // SPDX-License-Identifier: MIT
 
 #ifndef INCLUDE_NLOHMANN_JSON_FWD_HPP_
-    #define INCLUDE_NLOHMANN_JSON_FWD_HPP_
+#define INCLUDE_NLOHMANN_JSON_FWD_HPP_
 
-    #include <cstdint> // int64_t, uint64_t
-    #include <map> // map
-    #include <memory> // allocator
-    #include <string> // string
-    #include <vector> // vector
+#include <cstdint> // int64_t, uint64_t
+#include <map> // map
+#include <memory> // allocator
+#include <string> // string
+#include <vector> // vector
 
-    // #include <nlohmann/detail/abi_macros.hpp>
+// #include <nlohmann/detail/abi_macros.hpp>
 
 
-    /*!
-    @brief namespace for Niels Lohmann
-    @see https://github.com/nlohmann
-    @since version 1.0.0
-    */
-    NLOHMANN_JSON_NAMESPACE_BEGIN
+/*!
+@brief namespace for Niels Lohmann
+@see https://github.com/nlohmann
+@since version 1.0.0
+*/
+NLOHMANN_JSON_NAMESPACE_BEGIN
 
-    /*!
-    @brief default JSONSerializer template argument
+/*!
+@brief default JSONSerializer template argument
 
-    This serializer ignores the template arguments and uses ADL
-    ([argument-dependent lookup](https://en.cppreference.com/w/cpp/language/adl))
-    for serialization.
-    */
-    template<typename T = void, typename SFINAE = void>
-    struct adl_serializer;
-
-    /// a class to store JSON values
-    /// @sa https://json.nlohmann.me/api/basic_json/
-    template<template<typename U, typename V, typename... Args> class ObjectType =
-    std::map,
-    template<typename U, typename... Args> class ArrayType = std::vector,
-    class StringType = std::string, class BooleanType = bool,
-    class NumberIntegerType = std::int64_t,
-    class NumberUnsignedType = std::uint64_t,
-    class NumberFloatType = double,
-    template<typename U> class AllocatorType = std::allocator,
-    template<typename T, typename SFINAE = void> class JSONSerializer =
-    adl_serializer,
-    class BinaryType = std::vector<std::uint8_t>, // cppcheck-suppress syntaxError
-    class CustomBaseClass = void>
-    class basic_json;
-
-    /// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
-    /// @sa https://json.nlohmann.me/api/json_pointer/
-    template<typename RefStringType>
-    class json_pointer;
+This serializer ignores the template arguments and uses ADL
+([argument-dependent lookup](https://en.cppreference.com/w/cpp/language/adl))
+for serialization.
+*/
+template<typename T = void, typename SFINAE = void>
+struct adl_serializer;
+
+/// a class to store JSON values
+/// @sa https://json.nlohmann.me/api/basic_json/
+template<template<typename U, typename V, typename... Args> class ObjectType =
+         std::map,
+         template<typename U, typename... Args> class ArrayType = std::vector,
+         class StringType = std::string, class BooleanType = bool,
+         class NumberIntegerType = std::int64_t,
+         class NumberUnsignedType = std::uint64_t,
+         class NumberFloatType = double,
+         template<typename U> class AllocatorType = std::allocator,
+         template<typename T, typename SFINAE = void> class JSONSerializer =
+         adl_serializer,
+         class BinaryType = std::vector<std::uint8_t>, // cppcheck-suppress syntaxError
+         class CustomBaseClass = void>
+class basic_json;
 
-    /*!
-    @brief default specialization
-    @sa https://json.nlohmann.me/api/json/
-    */
-    using json = basic_json<>;
+/// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
+/// @sa https://json.nlohmann.me/api/json_pointer/
+template<typename RefStringType>
+class json_pointer;
 
-    /// @brief a minimal map-like container that preserves insertion order
-    /// @sa https://json.nlohmann.me/api/ordered_map/
-    template<class Key, class T, class IgnoredLess, class Allocator>
-    struct ordered_map;
+/*!
+@brief default specialization
+@sa https://json.nlohmann.me/api/json/
+*/
+using json = basic_json<>;
 
-    /// @brief specialization that maintains the insertion order of object keys
-    /// @sa https://json.nlohmann.me/api/ordered_json/
-    using ordered_json = basic_json<nlohmann::ordered_map>;
+/// @brief a minimal map-like container that preserves insertion order
+/// @sa https://json.nlohmann.me/api/ordered_map/
+template<class Key, class T, class IgnoredLess, class Allocator>
+struct ordered_map;
 
-    NLOHMANN_JSON_NAMESPACE_END
+/// @brief specialization that maintains the insertion order of object keys
+/// @sa https://json.nlohmann.me/api/ordered_json/
+using ordered_json = basic_json<nlohmann::ordered_map>;
+
+NLOHMANN_JSON_NAMESPACE_END
 
 #endif  // INCLUDE_NLOHMANN_JSON_FWD_HPP_
 
@@ -5427,7 +5427,7 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 // #include <nlohmann/detail/macro_scope.hpp>
-// JSON_HAS_CPP_17
+ // JSON_HAS_CPP_17
 #ifdef JSON_HAS_CPP_17
     #include <optional> // optional
 #endif
@@ -20255,10 +20255,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         const bool allow_exceptions = true,
         const bool ignore_comments = false,
         const bool ignore_trailing_commas = false
-                                 )
+    )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
-            std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
+               std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
     }
 
   private:
@@ -20956,8 +20956,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                detail::enable_if_t <
                    !detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value, int > = 0 >
     basic_json(CompatibleType && val) noexcept(noexcept( // NOLINT(bugprone-forwarding-reference-overload,bugprone-exception-escape)
-            JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
-                                       std::forward<CompatibleType>(val))))
+                JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
+                                           std::forward<CompatibleType>(val))))
     {
         JSONSerializer<U>::to_json(*this, std::forward<CompatibleType>(val));
         set_parents();
@@ -21751,7 +21751,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    detail::has_from_json<basic_json_t, ValueType>::value,
                    int > = 0 >
     ValueType get_impl(detail::priority_tag<0> /*unused*/) const noexcept(noexcept(
-            JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
     {
         auto ret = ValueType();
         JSONSerializer<ValueType>::from_json(*this, ret);
@@ -21793,7 +21793,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    detail::has_non_default_from_json<basic_json_t, ValueType>::value,
                    int > = 0 >
     ValueType get_impl(detail::priority_tag<1> /*unused*/) const noexcept(noexcept(
-            JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>())))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>())))
     {
         return JSONSerializer<ValueType>::from_json(*this);
     }
@@ -21943,7 +21943,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    detail::has_from_json<basic_json_t, ValueType>::value,
                    int > = 0 >
     ValueType & get_to(ValueType& v) const noexcept(noexcept(
-            JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), v)))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), v)))
     {
         JSONSerializer<ValueType>::from_json(*this, v);
         return v;
diff --git a/tests/src/unit-conversions.cpp b/tests/src/unit-conversions.cpp
index 83d7e8d7c6..e17f604053 100644
--- a/tests/src/unit-conversions.cpp
+++ b/tests/src/unit-conversions.cpp
@@ -1683,7 +1683,7 @@ TEST_CASE("std::filesystem::path")
 }
 #endif
 
-#ifndef JSON_USE_IMPLICIT_CONVERSIONS
+#if !JSON_USE_IMPLICIT_CONVERSIONS
 TEST_CASE("std::optional")
 {
     SECTION("null")