From 85f6ecc7a01dcd9428dd365cbabc568b96c4815e Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Sun, 19 Oct 2025 02:05:21 +0500
Subject: [PATCH] Add format_as support for std::variant and std::expected
 formatters (#4575)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/format.h |  8 ++++++++
 include/fmt/ranges.h |  8 --------
 include/fmt/std.h    | 28 ++++++++++++---------------
 test/std-test.cc     | 46 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 66 insertions(+), 24 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 0cd3a232..4a653007 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -763,6 +763,14 @@ template <typename T> struct allocator : private std::decay<void> {
   }
 };
 
+template <typename Formatter>
+FMT_CONSTEXPR auto maybe_set_debug_format(Formatter& f, bool set)
+    -> decltype(f.set_debug_format(set)) {
+  f.set_debug_format(set);
+}
+template <typename Formatter>
+FMT_CONSTEXPR void maybe_set_debug_format(Formatter&, ...) {}
+
 }  // namespace detail
 
 FMT_BEGIN_EXPORT
diff --git a/include/fmt/ranges.h b/include/fmt/ranges.h
index 0823cbf2..36b38e29 100644
--- a/include/fmt/ranges.h
+++ b/include/fmt/ranges.h
@@ -241,14 +241,6 @@ using range_reference_type =
 template <typename Range>
 using uncvref_type = remove_cvref_t<range_reference_type<Range>>;
 
-template <typename Formatter>
-FMT_CONSTEXPR auto maybe_set_debug_format(Formatter& f, bool set)
-    -> decltype(f.set_debug_format(set)) {
-  f.set_debug_format(set);
-}
-template <typename Formatter>
-FMT_CONSTEXPR void maybe_set_debug_format(Formatter&, ...) {}
-
 template <typename T>
 struct range_format_kind_
     : std::integral_constant<range_format,
diff --git a/include/fmt/std.h b/include/fmt/std.h
index 1127feaa..3fe795bf 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -111,12 +111,17 @@ void write_escaped_path(basic_memory_buffer<Char>& quoted,
 #endif  // FMT_CPP_LIB_FILESYSTEM
 
 #if defined(__cpp_lib_expected) || FMT_CPP_LIB_VARIANT
-template <typename Char, typename OutputIt, typename T>
-auto write_escaped_alternative(OutputIt out, const T& v) -> OutputIt {
+
+template <typename Char, typename OutputIt, typename T, typename FormatContext>
+auto write_escaped_alternative(OutputIt out, const T& v, FormatContext& ctx)
+    -> OutputIt {
   if constexpr (has_to_string_view<T>::value)
     return write_escaped_string<Char>(out, detail::to_string_view(v));
   if constexpr (std::is_same_v<T, Char>) return write_escaped_char(out, v);
-  return write<Char>(out, v);
+
+  formatter<std::remove_cv_t<T>, Char> underlying;
+  maybe_set_debug_format(underlying, true);
+  return underlying.format(v, ctx);
 }
 #endif
 
@@ -382,18 +387,9 @@ struct formatter<std::optional<T>, Char,
   static constexpr basic_string_view<Char> none =
       detail::string_literal<Char, 'n', 'o', 'n', 'e'>{};
 
-  template <class U>
-  FMT_CONSTEXPR static auto maybe_set_debug_format(U& u, bool set)
-      -> decltype(u.set_debug_format(set)) {
-    u.set_debug_format(set);
-  }
-
-  template <class U>
-  FMT_CONSTEXPR static void maybe_set_debug_format(U&, ...) {}
-
  public:
   FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) {
-    maybe_set_debug_format(underlying_, true);
+    detail::maybe_set_debug_format(underlying_, true);
     return underlying_.parse(ctx);
   }
 
@@ -429,10 +425,10 @@ struct formatter<std::expected<T, E>, Char,
     if (value.has_value()) {
       out = detail::write<Char>(out, "expected(");
       if constexpr (!std::is_void<T>::value)
-        out = detail::write_escaped_alternative<Char>(out, *value);
+        out = detail::write_escaped_alternative<Char>(out, *value, ctx);
     } else {
       out = detail::write<Char>(out, "unexpected(");
-      out = detail::write_escaped_alternative<Char>(out, value.error());
+      out = detail::write_escaped_alternative<Char>(out, value.error(), ctx);
     }
     *out++ = ')';
     return out;
@@ -496,7 +492,7 @@ struct formatter<Variant, Char,
     FMT_TRY {
       std::visit(
           [&](const auto& v) {
-            out = detail::write_escaped_alternative<Char>(out, v);
+            out = detail::write_escaped_alternative<Char>(out, v, ctx);
           },
           value);
     }
diff --git a/test/std-test.cc b/test/std-test.cc
index 4bd8abce..3fdf0532 100644
--- a/test/std-test.cc
+++ b/test/std-test.cc
@@ -197,7 +197,33 @@ class my_class {
     return fmt::to_string(elm.av);
   }
 };
+
+class my_class_int {
+ public:
+  int av;
+
+ private:
+  friend auto format_as(const my_class_int& elm) -> int { return elm.av; }
+};
 }  // namespace my_nso
+
+TEST(std_test, expected_format_as) {
+#ifdef __cpp_lib_expected
+  EXPECT_EQ(
+      fmt::format(
+          "{}", std::expected<my_nso::my_number, int>{my_nso::my_number::one}),
+      "expected(\"first\")");
+  EXPECT_EQ(
+      fmt::format("{}",
+                  std::expected<my_nso::my_class, int>{my_nso::my_class{7}}),
+      "expected(\"7\")");
+  EXPECT_EQ(fmt::format("{}",
+                        std::expected<my_nso::my_class_int, int>{
+                            my_nso::my_class_int{8}}),
+            "expected(8)");
+#endif
+}
+
 TEST(std_test, optional_format_as) {
 #ifdef __cpp_lib_optional
   EXPECT_EQ(fmt::format("{}", std::optional<my_nso::my_number>{}), "none");
@@ -206,6 +232,8 @@ TEST(std_test, optional_format_as) {
   EXPECT_EQ(fmt::format("{}", std::optional<my_nso::my_class>{}), "none");
   EXPECT_EQ(fmt::format("{}", std::optional{my_nso::my_class{7}}),
             "optional(\"7\")");
+  EXPECT_EQ(fmt::format("{}", std::optional{my_nso::my_class_int{8}}),
+            "optional(8)");
 #endif
 }
 
@@ -275,6 +303,24 @@ TEST(std_test, variant) {
 #endif
 }
 
+TEST(std_test, variant_format_as) {
+#ifdef __cpp_lib_variant
+
+  EXPECT_EQ(fmt::format("{}", std::variant<my_nso::my_number>{}),
+            "variant(\"first\")");
+  EXPECT_EQ(fmt::format(
+                "{}", std::variant<my_nso::my_number>{my_nso::my_number::one}),
+            "variant(\"first\")");
+  EXPECT_EQ(
+      fmt::format("{}", std::variant<my_nso::my_class>{my_nso::my_class{7}}),
+      "variant(\"7\")");
+  EXPECT_EQ(
+      fmt::format("{}",
+                  std::variant<my_nso::my_class_int>{my_nso::my_class_int{8}}),
+      "variant(8)");
+#endif
+}
+
 TEST(std_test, error_code) {
   auto& generic = std::generic_category();
   EXPECT_EQ(fmt::format("{}", std::error_code(42, generic)), "generic:42");