Fix FP formatting to a non-back_insert_iterator with sign & numeric alignment (#756)

include/fmt/format.h

@@ -2927,7 +2927,8 @@ void basic_writer<Range>::write_double(T value, const format_specs &spec) {
   align_spec as = spec;
   if (spec.align() == ALIGN_NUMERIC) {
     if (sign) {
-      *reserve(1) = sign;
+      auto &&it = reserve(1);
+      *it++ = sign;
       sign = 0;
       if (as.width_)
         --as.width_;

test/format-test.cc

@@ -232,6 +232,12 @@ TEST(FormatToTest, Format) {
   EXPECT_EQ("part1part2", s);
 }
 
+TEST(FormatToTest, FormatToNonbackInsertIteratorWithSignAndNumericAlignment) {
+  char buffer[16] = {};
+  fmt::format_to(buffer, "{: =+}", 42.0);
+  EXPECT_STREQ("+42", buffer);
+}
+
 TEST(FormatterTest, Escape) {
   EXPECT_EQ("{", format("{{"));
   EXPECT_EQ("before {", format("before {{"));