From a05b22ddf3fe9d6a6658313b64071b5f25576ef2 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Tue, 29 Sep 2026 23:17:33 +0200
Subject: [PATCH] Let to_json(std::optional<T>) propagate exceptions from T's
 to_json

The overload was marked noexcept even though its body assigns *opt to
the JSON value, which calls T's to_json (or allocates for std::string,
std::vector, or json). Any exception from there -- a user-defined
to_json reporting an error, or std::bad_alloc -- called std::terminate()
instead of propagating. The noexcept also made basic_json's converting
constructor noexcept(true) for std::optional<T>, so json j = opt; could
not report the error either.

Fixes #5642.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---
 .../nlohmann/detail/conversions/to_json.hpp   |  2 +-
 single_include/nlohmann/json.hpp              |  2 +-
 tests/src/unit-conversions.cpp                | 23 +++++++++++++++++++
 3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/nlohmann/detail/conversions/to_json.hpp b/include/nlohmann/detail/conversions/to_json.hpp
index 491bb9873..d77381249 100644
--- a/include/nlohmann/detail/conversions/to_json.hpp
+++ b/include/nlohmann/detail/conversions/to_json.hpp
@@ -287,7 +287,7 @@ struct external_constructor<value_t::object>
 #ifdef JSON_HAS_CPP_17
 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
+void to_json(BasicJsonType& j, const std::optional<T>& opt)
 {
     if (opt.has_value())
     {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 576498738..83b6f10be 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -6816,7 +6816,7 @@ struct external_constructor<value_t::object>
 #ifdef JSON_HAS_CPP_17
 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
+void to_json(BasicJsonType& j, const std::optional<T>& opt)
 {
     if (opt.has_value())
     {
diff --git a/tests/src/unit-conversions.cpp b/tests/src/unit-conversions.cpp
index 90d972f71..6e0c73b48 100644
--- a/tests/src/unit-conversions.cpp
+++ b/tests/src/unit-conversions.cpp
@@ -1826,6 +1826,16 @@ TEST_CASE("std::u8string")
 #endif
 #endif
 
+// a type whose to_json reports an error by throwing, used below to check that
+// converting a std::optional<T> to JSON propagates an exception thrown while
+// converting its contained value instead of calling std::terminate (#5642)
+struct throwing_to_json_type {};
+
+void to_json(json&, const throwing_to_json_type&)
+{
+    throw std::runtime_error("cannot serialize throwing_to_json_type");
+}
+
 TEST_CASE("std::optional")
 {
     SECTION("null")
@@ -1908,6 +1918,19 @@ TEST_CASE("std::optional")
         CHECK(json(opt_object) == j_object);
         CHECK(std::map<std::string, std::optional<int>>(j_object) == opt_object);
     }
+
+#if !defined(JSON_NOEXCEPTION)
+    SECTION("exception from contained value's to_json propagates (#5642)")
+    {
+        // to_json(BasicJsonType&, const std::optional<T>&) must not be
+        // noexcept: it calls T's to_json, which may throw (a user-defined
+        // to_json that reports an error, or std::bad_alloc for T =
+        // std::string/vector/json). Before the fix, this called
+        // std::terminate() instead of letting the exception propagate.
+        const std::optional<throwing_to_json_type> opt = throwing_to_json_type{};
+        CHECK_THROWS_WITH_AS(json(opt), "cannot serialize throwing_to_json_type", std::runtime_error&);
+    }
+#endif
 }
 #endif