fix std::filesystem::path regression (#3073)

* meta: rework is_compatible/is_constructible_string_type

These type traits performed an incorrect and insufficient check.

Converting to a std::filesystem::path used to work by accident thanks to
these brittle constraints, but the clean-up performed in #3020 broke them.

* support std::filesystem::path

Fixes #3070

include/nlohmann/detail/conversions/from_json.hpp

@@ -19,6 +19,10 @@
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
+#ifdef JSON_HAS_CPP_17
+    #include <filesystem>
+#endif
+
 namespace nlohmann
 {
 namespace detail
@@ -169,7 +173,7 @@ void from_json(const BasicJsonType& j, std::valarray<T>& l)
 }
 
 template<typename BasicJsonType, typename T, std::size_t N>
-auto from_json(const BasicJsonType& j, T (&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+auto from_json(const BasicJsonType& j, T (&arr)[N])  // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
 -> decltype(j.template get<T>(), void())
 {
     for (std::size_t i = 0; i < N; ++i)
@@ -444,6 +448,18 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
     }
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType>
+void from_json(const BasicJsonType& j, std::filesystem::path& p)
+{
+    if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
+    {
+        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
+    }
+    p = *j.template get_ptr<const typename BasicJsonType::string_t*>();
+}
+#endif
+
 struct from_json_fn
 {
     template<typename BasicJsonType, typename T>

include/nlohmann/detail/conversions/to_json.hpp

@@ -9,11 +9,16 @@
 #include <valarray> // valarray
 #include <vector> // vector
 
+#include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/iterators/iteration_proxy.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
+#ifdef JSON_HAS_CPP_17
+    #include <filesystem>
+#endif
+
 namespace nlohmann
 {
 namespace detail
@@ -386,6 +391,14 @@ void to_json(BasicJsonType& j, const T& t)
     to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType>
+void to_json(BasicJsonType& j, const std::filesystem::path& p)
+{
+    j = p.string();
+}
+#endif
+
 struct to_json_fn
 {
     template<typename BasicJsonType, typename T>