Fix ambiguous formatter<std::optional<T>> between fmt/std.h and fmt/ranges.h on C++26 (P3168R2) (#4761)

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

Author: phprus

include/fmt/ranges.h

@@ -129,6 +129,13 @@ template <typename T> class is_tuple_like_ {
       !std::is_void<decltype(check<T>(nullptr))>::value;
 };
 
+template <typename T, typename _ = void>
+struct is_optional_like_ : std::false_type {};
+template <typename T>
+struct is_optional_like_<T, void_t<decltype(std::declval<T>().has_value()),
+                                   decltype(std::declval<T>().value())>>
+    : std::true_type {};
+
 // Check for integer_sequence
 #if defined(__cpp_lib_integer_sequence) || FMT_MSC_VERSION >= 1900
 template <typename T, T... N>
@@ -343,8 +350,9 @@ struct formatter<Tuple, Char,
 
 FMT_EXPORT
 template <typename T, typename Char> struct is_range {
-  static constexpr bool value =
-      detail::is_range_<T>::value && !detail::has_to_string_view<T>::value;
+  static constexpr bool value = detail::is_range_<T>::value &&
+                                !detail::is_optional_like_<T>::value &&
+                                !detail::has_to_string_view<T>::value;
 };
 
 namespace detail {

test/ranges-test.cc

@@ -271,6 +271,23 @@ TEST(ranges_test, disabled_range_formatting_of_path) {
             fmt::range_format::disabled);
 }
 
+template <typename T> struct optional_like {
+  auto begin() const -> const T*;
+  auto end() const -> const T*;
+
+  bool has_value() const noexcept;
+  T& value() &;
+  const T& value() const&;
+  T&& value() &&;
+  const T&& value() const&&;
+};
+
+TEST(ranges_test, disabled_range_formatting_of_optional) {
+  // (C++26) Has a range support for std::optional.
+  EXPECT_EQ((fmt::range_format_kind<optional_like<int>, char>::value),
+            fmt::range_format::disabled);
+}
+
 struct vector_string : std::vector<char> {
   using base = std::vector<char>;
   using base::base;