From 22d50c1a9c23a65ce56086a3eb74dbe29aede957 Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Tue, 3 Sep 2024 21:44:10 +0500
Subject: [PATCH] Add support formatting std::expected<void, E>

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/std.h | 6 ++++--
 test/std-test.cc  | 2 ++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/fmt/std.h b/include/fmt/std.h
index 28460846..00eee38f 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -279,7 +279,8 @@ FMT_BEGIN_NAMESPACE
 FMT_EXPORT
 template <typename T, typename E, typename Char>
 struct formatter<std::expected<T, E>, Char,
-                 std::enable_if_t<is_formattable<T, Char>::value &&
+                 std::enable_if_t<(std::is_void<T>::value ||
+                                   is_formattable<T, Char>::value) &&
                                   is_formattable<E, Char>::value>> {
   FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) -> const Char* {
     return ctx.begin();
@@ -292,7 +293,8 @@ struct formatter<std::expected<T, E>, Char,
 
     if (value.has_value()) {
       out = detail::write<Char>(out, "expected(");
-      out = detail::write_escaped_alternative<Char>(out, *value);
+      if constexpr (!std::is_void<T>::value)
+        out = detail::write_escaped_alternative<Char>(out, *value);
     } else {
       out = detail::write<Char>(out, "unexpected(");
       out = detail::write_escaped_alternative<Char>(out, value.error());
diff --git a/test/std-test.cc b/test/std-test.cc
index 42444181..5dad5fb2 100644
--- a/test/std-test.cc
+++ b/test/std-test.cc
@@ -140,6 +140,7 @@ TEST(std_test, optional) {
 
 TEST(std_test, expected) {
 #ifdef __cpp_lib_expected
+  EXPECT_EQ(fmt::format("{}", std::expected<void, int>{}), "expected()");
   EXPECT_EQ(fmt::format("{}", std::expected<int, int>{1}), "expected(1)");
   EXPECT_EQ(fmt::format("{}", std::expected<int, int>{std::unexpected(1)}),
             "unexpected(1)");
@@ -163,6 +164,7 @@ TEST(std_test, expected) {
   EXPECT_FALSE(
       (fmt::is_formattable<std::expected<int, unformattable2>>::value));
   EXPECT_TRUE((fmt::is_formattable<std::expected<int, int>>::value));
+  EXPECT_TRUE((fmt::is_formattable<std::expected<void, int>>::value));
 #endif
 }