Add support formatting std::expected<void, E>

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
fmtlib · Sep 3, 2024 · a74aee3 · a74aee3
1 parent 7a6a2a7
commit a74aee3
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Showing 2 changed files with 19 additions and 4 deletions.
diff --git a/include/fmt/std.h b/include/fmt/std.h
@@ -277,10 +277,22 @@ FMT_END_NAMESPACE
 FMT_BEGIN_NAMESPACE
 
 FMT_EXPORT
+
+namespace detail {
+
+template <typename T, typename Char>
+struct is_expected_formattable : is_formattable<T, Char> {};
+
+template <typename Char>
+struct is_expected_formattable<void, Char> : std::true_type {};
+
+}  // namespace detail
+
 template <typename T, typename E, typename Char>
-struct formatter<std::expected<T, E>, Char,
-                 std::enable_if_t<is_formattable<T, Char>::value &&
-                                  is_formattable<E, Char>::value>> {
+struct formatter<
+    std::expected<T, E>, Char,
+    std::enable_if_t<detail::is_expected_formattable<T, Char>::value &&
+                     is_formattable<E, Char>::value>> {
   FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) -> const Char* {
     return ctx.begin();
   }
@@ -292,7 +304,8 @@ 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
@@ -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
 }