From 4b53fc202447a945818008ab5b9b7c473ddb3748 Mon Sep 17 00:00:00 2001
From: Luiz Siqueira <indiosmo@gmail.com>
Date: Thu, 30 Apr 2026 15:51:38 -0300
Subject: [PATCH] Fix std.h/ranges.h ambiguity for optional in c++26

---
 include/fmt/ranges.h | 13 +++++++++++++
 test/ranges-test.cc  | 12 ++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/include/fmt/ranges.h b/include/fmt/ranges.h
index d7dbc168..26f7895d 100644
--- a/include/fmt/ranges.h
+++ b/include/fmt/ranges.h
@@ -14,6 +14,13 @@
 #  include <tuple>
 #  include <type_traits>
 #  include <utility>
+
+// Check FMT_CPLUSPLUS to suppress a bogus warning in MSVC.
+#  if FMT_CPLUSPLUS >= 201703L
+#    if FMT_HAS_INCLUDE(<optional>)
+#      include <optional>
+#    endif
+#  endif
 #endif
 
 #include "format.h"
@@ -487,6 +494,12 @@ struct range_format_kind
           is_range<T, Char>::value, detail::range_format_kind_<T>,
           std::integral_constant<range_format, range_format::disabled>> {};
 
+#if defined(__cpp_lib_optional_range_support)
+template <typename T, typename Char>
+struct range_format_kind<std::optional<T>, Char>
+    : std::integral_constant<range_format, range_format::disabled> {};
+#endif
+
 template <typename R, typename Char>
 struct formatter<
     R, Char,
diff --git a/test/ranges-test.cc b/test/ranges-test.cc
index bd161d75..2f35f293 100644
--- a/test/ranges-test.cc
+++ b/test/ranges-test.cc
@@ -20,6 +20,10 @@
 #  include <ranges>
 #endif
 
+#if FMT_CPLUSPLUS > 201703L && FMT_HAS_INCLUDE(<optional>)
+#  include <optional>
+#endif
+
 #include "fmt/format.h"
 #include "gtest/gtest.h"
 
@@ -271,6 +275,14 @@ TEST(ranges_test, disabled_range_formatting_of_path) {
             fmt::range_format::disabled);
 }
 
+# if defined(__cpp_lib_optional_range_support)
+TEST(ranges_test, disabled_range_formatting_of_optional) {
+  // Range format of optional is disabled to avoid ambiguity with fmt/std.h.
+  EXPECT_EQ((fmt::range_format_kind<std::optional<int>, char>::value),
+            fmt::range_format_disabled);
+}
+#endif
+
 struct vector_string : std::vector<char> {
   using base = std::vector<char>;
   using base::base;