From 2234cd2d4040b3bdedafaaf2e96fa016d505745c Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Mon, 5 Oct 2026 20:08:17 +0200
Subject: [PATCH] Fix std::terminate when converting std::optional with MSVC
 2017

MSVC 2017 evaluates std::is_nothrow_assignable<json&, const T&> as true
even if T's to_json throws, so to_json(json&, const std::optional<T>&)
was noexcept there and the exception from #5642's test called
std::terminate instead of propagating. Make that conversion never
noexcept on MSVC 2017; all other compilers keep the exact condition.
The static_asserts on the condition are skipped for MSVC 2017; the
runtime check that the exception propagates still runs there.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---
 include/nlohmann/detail/conversions/to_json.hpp | 13 ++++++++++++-
 single_include/nlohmann/json.hpp                | 13 ++++++++++++-
 tests/src/unit-conversions2.cpp                 |  6 +++---
 3 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/include/nlohmann/detail/conversions/to_json.hpp b/include/nlohmann/detail/conversions/to_json.hpp
index d7197f323..f3b4994f1 100644
--- a/include/nlohmann/detail/conversions/to_json.hpp
+++ b/include/nlohmann/detail/conversions/to_json.hpp
@@ -287,9 +287,20 @@ struct external_constructor<value_t::object>
 /////////////
 
 #ifdef JSON_HAS_CPP_17
+// whether storing the value of a std::optional<T> cannot throw; MSVC 2017
+// evaluates std::is_nothrow_assignable as true even if T's to_json throws, so
+// the exception would call std::terminate (#5642)
+#if defined(_MSC_VER) && !defined(__clang__) && _MSC_VER < 1920
+    template<typename BasicJsonType, typename T>
+    using is_nothrow_optional_to_json = std::false_type;
+#else
+    template<typename BasicJsonType, typename T>
+    using is_nothrow_optional_to_json = std::is_nothrow_assignable<BasicJsonType&, const T&>;
+#endif
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_nothrow_assignable<BasicJsonType&, const T&>::value)
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(is_nothrow_optional_to_json<BasicJsonType, T>::value)
 {
     if (opt.has_value())
     {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 765095dcb..d0a651093 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -7081,9 +7081,20 @@ struct external_constructor<value_t::object>
 /////////////
 
 #ifdef JSON_HAS_CPP_17
+// whether storing the value of a std::optional<T> cannot throw; MSVC 2017
+// evaluates std::is_nothrow_assignable as true even if T's to_json throws, so
+// the exception would call std::terminate (#5642)
+#if defined(_MSC_VER) && !defined(__clang__) && _MSC_VER < 1920
+    template<typename BasicJsonType, typename T>
+    using is_nothrow_optional_to_json = std::false_type;
+#else
+    template<typename BasicJsonType, typename T>
+    using is_nothrow_optional_to_json = std::is_nothrow_assignable<BasicJsonType&, const T&>;
+#endif
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_nothrow_assignable<BasicJsonType&, const T&>::value)
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(is_nothrow_optional_to_json<BasicJsonType, T>::value)
 {
     if (opt.has_value())
     {
diff --git a/tests/src/unit-conversions2.cpp b/tests/src/unit-conversions2.cpp
index ed3103484..c543e4ff8 100644
--- a/tests/src/unit-conversions2.cpp
+++ b/tests/src/unit-conversions2.cpp
@@ -853,11 +853,11 @@ TEST_CASE("std::optional")
         CHECK_THROWS_WITH_AS(json(opt), "cannot serialize throwing_to_json_type", std::runtime_error&);
 
         // the conversion is noexcept exactly when converting the contained value is
-        // (MSVC 2017 evaluates the conditional noexcept of this conversion as true)
-#if !defined(_MSC_VER) || _MSC_VER >= 1920
+        // (except with MSVC 2017, where it is never noexcept, see to_json.hpp)
+#if !defined(_MSC_VER) || defined(__clang__) || _MSC_VER >= 1920
         static_assert(!std::is_nothrow_constructible<json, const std::optional<throwing_to_json_type>&>::value);
-#endif
         static_assert(std::is_nothrow_constructible<json, const std::optional<int>&>::value);
+#endif
     }
 #endif
 }