std::filesystem::path is not format as a range

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

include/fmt/ranges.h

@@ -59,6 +59,17 @@ template <typename T> class is_std_string_like {
       !std::is_void<decltype(check<T>(nullptr))>::value;
 };
 
+// Returns true if T has a std::filesystem::path-like interface.
+template <typename T> class is_std_filesystem_path_like {
+  template <typename U>
+  static auto check(U* p) -> decltype((void)U::preferred_separator, int());
+  template <typename> static void check(...);
+
+ public:
+  static constexpr const bool value =
+      !std::is_void<decltype(check<T>(nullptr))>::value;
+};
+
 template <typename Char>
 struct is_std_string_like<fmt::basic_string_view<Char>> : std::true_type {};
 
@@ -383,7 +394,8 @@ template <typename T, typename Char> struct is_range {
   static constexpr const bool value =
       detail::is_range_<T>::value && !detail::is_std_string_like<T>::value &&
       !std::is_convertible<T, std::basic_string<Char>>::value &&
-      !std::is_convertible<T, detail::std_string_view<Char>>::value;
+      !std::is_convertible<T, detail::std_string_view<Char>>::value &&
+      !detail::is_std_filesystem_path_like<T>::value;
 };
 
 namespace detail {

test/ranges-test.cc

@@ -179,15 +179,18 @@ TEST(ranges_test, format_to) {
   EXPECT_STREQ(buf, "[1, 2, 3]");
 }
 
-struct path_like {
+template <typename Char> struct path_like {
   const path_like* begin() const;
   const path_like* end() const;
 
-  operator std::string() const;
+  operator std::basic_string<Char>() const;
+
+  static constexpr const Char preferred_separator = Char();
 };
 
 TEST(ranges_test, path_like) {
-  EXPECT_FALSE((fmt::is_range<path_like, char>::value));
+  EXPECT_FALSE((fmt::is_range<path_like<char>, char>::value));
+  EXPECT_FALSE((fmt::is_range<path_like<wchar_t>, char>::value));
 }
 
 // A range that provides non-const only begin()/end() to test fmt::join

test/std-test.cc

@@ -46,6 +46,12 @@ TEST(ranges_std_test, format_vector_path) {
 #endif
 }
 
+TEST(ranges_std_test, path) {
+#ifdef __cpp_lib_filesystem
+  EXPECT_FALSE((fmt::is_range<std::filesystem::path, char>::value));
+#endif
+}
+
 TEST(std_test, thread_id) {
   EXPECT_FALSE(fmt::format("{}", std::this_thread::get_id()).empty());
 }