Add optional support (#3303)

doc/api.rst

@@ -508,6 +508,7 @@ Standard Library Types Formatting
 * `std::thread::id <https://en.cppreference.com/w/cpp/thread/thread/id>`_
 * `std::monostate <https://en.cppreference.com/w/cpp/utility/variant/monostate>`_
 * `std::variant <https://en.cppreference.com/w/cpp/utility/variant/variant>`_
+* `std::optional <https://en.cppreference.com/w/cpp/utility/optional>`_
 
 Formatting Variants
 -------------------

include/fmt/std.h

@@ -29,6 +29,9 @@
 #  if FMT_HAS_INCLUDE(<variant>)
 #    include <variant>
 #  endif
+#  if FMT_HAS_INCLUDE(<optional>)
+#    include <optional>
+#  endif
 #endif
 
 // GCC 4 does not support FMT_HAS_INCLUDE.
@@ -91,6 +94,49 @@ template <typename Char>
 struct formatter<std::thread::id, Char> : basic_ostream_formatter<Char> {};
 FMT_END_NAMESPACE
 
+#ifdef __cpp_lib_optional
+FMT_BEGIN_NAMESPACE
+template <typename T, typename Char>
+struct formatter<std::optional<T>, Char,
+                 std::enable_if_t<is_formattable<T, Char>::value>> {
+ private:
+  formatter<T, Char> underlying_;
+  static constexpr basic_string_view<Char> optional =
+      detail::string_literal<Char, 'o', 'p', 't', 'i', 'o', 'n', 'a', 'l',
+                             '('>{};
+  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:
+  template <typename ParseContext> FMT_CONSTEXPR auto parse(ParseContext& ctx) {
+    maybe_set_debug_format(underlying_, true);
+    return underlying_.parse(ctx);
+  }
+
+  template <typename FormatContext>
+  auto format(std::optional<T> const& opt, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    if (!opt) return detail::write<Char>(ctx.out(), none);
+
+    auto out = ctx.out();
+    out = detail::write<Char>(out, optional);
+    ctx.advance_to(out);
+    out = underlying_.format(*opt, ctx);
+    return detail::write(out, ')');
+  }
+};
+FMT_END_NAMESPACE
+#endif  // __cpp_lib_optional
+
 #ifdef __cpp_lib_variant
 FMT_BEGIN_NAMESPACE
 template <typename Char> struct formatter<std::monostate, Char> {

test/std-test.cc

@@ -50,6 +50,34 @@ TEST(std_test, thread_id) {
   EXPECT_FALSE(fmt::format("{}", std::this_thread::get_id()).empty());
 }
 
+TEST(std_test, optional) {
+#ifdef __cpp_lib_optional
+  EXPECT_EQ(fmt::format("{}", std::optional<int>{}), "none");
+  EXPECT_EQ(fmt::format("{}", std::pair{1, "second"}), "(1, \"second\")");
+  EXPECT_EQ(fmt::format("{}", std::vector{std::optional{1}, std::optional{2},
+                                          std::optional{3}}),
+            "[optional(1), optional(2), optional(3)]");
+  EXPECT_EQ(
+      fmt::format("{}", std::optional<std::optional<const char*>>{{"nested"}}),
+      "optional(optional(\"nested\"))");
+  EXPECT_EQ(
+      fmt::format("{:<{}}", std::optional{std::string{"left aligned"}}, 30),
+      "optional(\"left aligned\"                )");
+  EXPECT_EQ(
+      fmt::format("{::d}", std::optional{std::vector{'h', 'e', 'l', 'l', 'o'}}),
+      "optional([104, 101, 108, 108, 111])");
+  EXPECT_EQ(fmt::format("{}", std::optional{std::string{"string"}}),
+            "optional(\"string\")");
+  EXPECT_EQ(fmt::format("{}", std::optional{'C'}), "optional(\'C\')");
+  EXPECT_EQ(fmt::format("{:.{}f}", std::optional{3.14}, 1), "optional(3.1)");
+
+  struct unformattable {};
+  EXPECT_FALSE((fmt::is_formattable<unformattable>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::optional<unformattable>>::value));
+  EXPECT_TRUE((fmt::is_formattable<std::optional<int>>::value));
+#endif
+}
+
 TEST(std_test, variant) {
 #ifdef __cpp_lib_variant
   EXPECT_EQ(fmt::format("{}", std::monostate{}), "monostate");

test/xchar-test.cc

@@ -16,6 +16,7 @@
 #include "fmt/color.h"
 #include "fmt/ostream.h"
 #include "fmt/ranges.h"
+#include "fmt/std.h"
 #include "gtest-extra.h"  // Contains
 #include "util.h"         // get_locale
 
@@ -588,4 +589,12 @@ TEST(locale_test, sign) {
   EXPECT_EQ(fmt::format(std::locale(), L"{:L}", -50), L"-50");
 }
 
+TEST(std_test_xchar, optional) {
+#  ifdef __cpp_lib_optional
+  EXPECT_EQ(fmt::format(L"{}", std::optional{L'C'}), L"optional(\'C\')");
+  EXPECT_EQ(fmt::format(L"{}", std::optional{std::wstring{L"wide string"}}),
+            L"optional(\"wide string\")");
+#  endif
+}
+
 #endif  // FMT_STATIC_THOUSANDS_SEPARATOR