Workaround a bug in MSVC's strftime (#965)

include/fmt/time.h

@@ -370,7 +370,7 @@ struct formatter<std::tm, Char> {
 
   template <typename FormatContext>
   auto format(const std::tm &tm, FormatContext &ctx) -> decltype(ctx.out()) {
-    internal::basic_buffer<Char> &buf = internal::get_container(ctx.out());
+    basic_memory_buffer<Char> buf;
     std::size_t start = buf.size();
     for (;;) {
       std::size_t size = buf.capacity() - start;
@@ -390,7 +390,7 @@ struct formatter<std::tm, Char> {
       const std::size_t MIN_GROWTH = 10;
       buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
     }
-    return ctx.out();
+    return std::copy(buf.begin(), buf.end(), ctx.out());
   }
 
   basic_memory_buffer<Char> tm_format;

test/time-test.cc

@@ -31,6 +31,14 @@ TEST(TimeTest, GrowBuffer) {
   fmt::format(s, *std::localtime(&t));
 }
 
+TEST(TimeTest, FormatToEmptyContainer) {
+  std::string s;
+  auto time = std::tm();
+  time.tm_sec = 42;
+  fmt::format_to(std::back_inserter(s), "{:%S}", time);
+  EXPECT_EQ(s, "42");
+}
+
 TEST(TimeTest, EmptyResult) {
   EXPECT_EQ("", fmt::format("{}", std::tm()));
 }