Add std::complex formatter

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

include/fmt/std.h

@@ -10,6 +10,7 @@
 
 #include <atomic>
 #include <bitset>
+#include <complex>
 #include <cstdlib>
 #include <exception>
 #include <memory>
@@ -585,5 +586,31 @@ struct formatter<std::atomic_flag, Char> : formatter<bool, Char> {
 };
 #endif  // __cpp_lib_atomic_flag_test
 
+FMT_EXPORT
+template <typename F, typename Char>
+struct formatter<std::complex<F>, Char> : nested_formatter<F, Char> {
+ private:
+  // Functor because C++11 doesn't support generic lambdas.
+  struct writer {
+    const formatter<std::complex<F>, Char>* f;
+    const std::complex<F>& c;
+
+    template <typename OutputIt>
+    FMT_CONSTEXPR auto operator()(OutputIt out) -> OutputIt {
+      auto format =
+          detail::string_literal<Char, '(', '{', '}', ',', '{', '}', ')'>{};
+      return fmt::format_to(out, basic_string_view<Char>(format),
+                            f->nested(c.real()), f->nested(c.imag()));
+    }
+  };
+
+ public:
+  template <typename FormatContext>
+  auto format(const std::complex<F>& c, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return this->write_padded(ctx, writer{this, c});
+  }
+};
+
 FMT_END_NAMESPACE
 #endif  // FMT_STD_H_