Improve std::complex formatter to be compatible with P2197R0 (fmtlib#3900)

Signed-off-by: Vladislav Shchapov <[email protected]>

Author: phprus

Diff for: 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 for: 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)");
 }