Add formatting support for <stdfloat> types

Provide format_as for std::float16_t and std::bfloat16_t and dedicated
std::complex formatters so fmt::print works with C++23 stdfloat types
(fixes #4725).

doc/api.md

@@ -514,6 +514,9 @@ chrono-format-specifications).
   https://en.cppreference.com/w/cpp/utility/source_location)
 - [`std::thread::id`](https://en.cppreference.com/w/cpp/thread/thread/id)
 - [`std::variant`](https://en.cppreference.com/w/cpp/utility/variant/variant)
+- C++23 [`std::float16_t`](https://en.cppreference.com/w/cpp/types/float16_t) /
+  [`std::bfloat16_t`](https://en.cppreference.com/w/cpp/types/bfloat16_t) and
+  `std::complex` of those types (via `format_as`, when `<stdfloat>` is available)
 
 ::: ptr(const std::unique_ptr<T, Deleter>&)
 

include/fmt/std.h

@@ -804,6 +804,55 @@ struct formatter<std::reference_wrapper<T>, Char,
   }
 };
 
+#if FMT_HAS_INCLUDE(<stdfloat>) && defined(__cpp_lib_stdfloat)
+#  include <stdfloat>
+
+inline auto format_as(std::float16_t value) -> float {
+  return static_cast<float>(value);
+}
+inline auto format_as(std::bfloat16_t value) -> float {
+  return static_cast<float>(value);
+}
+
+template <typename Char>
+struct formatter<std::complex<std::float16_t>, Char> {
+ private:
+  formatter<std::complex<float>, Char> underlying_;
+
+ public:
+  FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) -> const Char* {
+    return underlying_.parse(ctx);
+  }
+  template <typename FormatContext>
+  auto format(const std::complex<std::float16_t>& value, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return underlying_.format(
+        std::complex<float>(static_cast<float>(value.real()),
+                            static_cast<float>(value.imag())),
+        ctx);
+  }
+};
+
+template <typename Char>
+struct formatter<std::complex<std::bfloat16_t>, Char> {
+ private:
+  formatter<std::complex<float>, Char> underlying_;
+
+ public:
+  FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) -> const Char* {
+    return underlying_.parse(ctx);
+  }
+  template <typename FormatContext>
+  auto format(const std::complex<std::bfloat16_t>& value,
+              FormatContext& ctx) const -> decltype(ctx.out()) {
+    return underlying_.format(
+        std::complex<float>(static_cast<float>(value.real()),
+                            static_cast<float>(value.imag())),
+        ctx);
+  }
+};
+#endif  // stdfloat
+
 FMT_END_NAMESPACE
 
 #endif  // FMT_STD_H_

test/std-test.cc

@@ -111,6 +111,16 @@ TEST(std_test, complex) {
             "(1.00-2.20i)        ");
 }
 
+#if FMT_HAS_INCLUDE(<stdfloat>) && defined(__cpp_lib_stdfloat)
+#  include <stdfloat>
+TEST(std_test, stdfloat) {
+  std::float16_t f16 = std::float16_t(1.5f);
+  EXPECT_EQ(fmt::format("{}", f16), fmt::format("{}", 1.5f));
+  EXPECT_EQ(fmt::format("{}", std::complex<std::float16_t>(f16, std::float16_t(2.f))),
+            fmt::format("{}", std::complex<float>(1.5f, 2.f)));
+}
+#endif
+
 #ifdef __cpp_lib_source_location
 TEST(std_test, source_location) {
   std::source_location loc = std::source_location::current();