From bc535fe73a8af0cc89480ece28f24a93d59e2a89 Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Thu, 21 Mar 2024 01:31:32 +0500
Subject: [PATCH] Improve std::complex formatter to be compatible with P2197R0
 (#3900)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/std.h | 13 +++++++++----
 test/std-test.cc  |  1 +
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/fmt/std.h b/include/fmt/std.h
index 2d6012e3dc1f..7085cae8170a 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -595,10 +595,15 @@ struct formatter<std::complex<F>, Char> : nested_formatter<F, Char> {
 
     template <typename OutputIt>
     FMT_CONSTEXPR auto operator()(OutputIt out) -> OutputIt {
-      auto format = detail::string_literal<Char, '(', '{', '}', '+', '{', '}',
-                                           'i', ')'>{};
-      return fmt::format_to(out, basic_string_view<Char>(format),
-                            f->nested(c.real()), f->nested(c.imag()));
+      if (c.real() != 0) {
+        auto format_full = detail::string_literal<Char, '(', '{', '}', '+', '{',
+                                                  '}', 'i', ')'>{};
+        return fmt::format_to(out, basic_string_view<Char>(format_full),
+                              f->nested(c.real()), f->nested(c.imag()));
+      }
+      auto format_imag = detail::string_literal<Char, '{', '}', 'i'>{};
+      return fmt::format_to(out, basic_string_view<Char>(format_imag),
+                            f->nested(c.imag()));
     }
   };
 
diff --git a/test/std-test.cc b/test/std-test.cc
index 3fbcedf5764d..691b10976bd0 100644
--- a/test/std-test.cc
+++ b/test/std-test.cc
@@ -67,6 +67,7 @@ TEST(std_test, thread_id) {
 
 TEST(std_test, complex) {
   EXPECT_EQ(fmt::format("{}", std::complex<double>(1, 2.2)), "(1+2.2i)");
+  EXPECT_EQ(fmt::format("{}", std::complex<double>(0, 2.2)), "2.2i");
   EXPECT_EQ(fmt::format("{:>20.2f}", std::complex<double>(1, 2.2)),
             "        (1.00+2.20i)");
 }