Overwork project infrastructure (#5218)

* 📝 overwork project infrastructure

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 fix GCC16 issue

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 fix GCC16 issue

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 only build module for GCC

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 📝 fix documentation

Closes #5012: fix the error_handler_t::ignore wording

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 📝 fix documentation

Closes #4354: fix "Custom data source" example

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -86,6 +86,21 @@
 */
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
+namespace detail
+{
+// Trait to detect std::optional<T> specializations. It is defined here rather
+// than in type_traits.hpp so that adding the <optional> include does not change
+// the include order of the C++20 module's global module fragment (the include
+// is already pulled in by the conversion headers above); see src/modules/json.cppm.
+template<typename>
+struct is_std_optional : std::false_type {};
+
+#ifdef JSON_HAS_CPP_17
+template<typename T>
+struct is_std_optional<std::optional<T>> : std::true_type {};
+#endif
+}  // namespace detail
+
 /*!
 @brief a class to store JSON values
 
@@ -1937,6 +1952,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 #endif
 #if defined(JSON_HAS_CPP_17) && JSON_HAS_STATIC_RTTI
                                                 detail::negation<std::is_same<ValueType, std::any>>,
+#endif
+#if defined(JSON_HAS_CPP_17)
+                                                // std::optional<T> can construct itself from basic_json; excluding it
+                                                // here avoids an ambiguity with that constructor (e.g., under C++26)
+                                                detail::negation<detail::is_std_optional<ValueType>>,
 #endif
                                                 detail::is_detected_lazy<detail::get_template_function, const basic_json_t&, ValueType>
                                                 >::value, int >::type = 0 >