Fix format_to + FMT_STRING for wide character type (#3931)

Added overload that takes a wformat_string.
Fixes issue #3925.

Author: hmbj

Diff for: include/fmt/xchar.h

@@ -141,6 +141,13 @@ auto format(wformat_string<T...> fmt, T&&... args) -> std::wstring {
   return vformat(fmt::wstring_view(fmt), fmt::make_wformat_args(args...));
 }
 
+template <typename OutputIt, typename... T>
+auto format_to(OutputIt out, wformat_string<T...> fmt, T&&... args)
+    -> OutputIt {
+  return vformat_to(out, fmt::wstring_view(fmt),
+                    fmt::make_wformat_args(args...));
+}
+
 // Pass char_t as a default template parameter instead of using
 // std::basic_string<char_t<S>> to reduce the symbol size.
 template <typename S, typename... T,
@@ -186,8 +193,9 @@ auto vformat_to(OutputIt out, const S& format_str,
 
 template <typename OutputIt, typename S, typename... T,
           typename Char = detail::format_string_char_t<S>,
-          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value&&
-                            detail::is_exotic_char<Char>::value)>
+          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value &&
+                        !std::is_same<Char, char>::value &&
+                        !std::is_same<Char, wchar_t>::value)>
 inline auto format_to(OutputIt out, const S& fmt, T&&... args) -> OutputIt {
   return vformat_to(out, detail::to_string_view(fmt),
                     fmt::make_format_args<buffered_context<Char>>(args...));

Diff for: test/xchar-test.cc

@@ -195,6 +195,12 @@ TEST(xchar_test, format_to) {
   EXPECT_STREQ(buf.data(), L"42");
 }
 
+TEST(xchar_test, compile_time_string_format_to) {
+  std::wstring ws;
+  fmt::format_to(std::back_inserter(ws), FMT_STRING(L"{}"), 42);
+  EXPECT_EQ(L"42", ws);
+}
+
 TEST(xchar_test, vformat_to) {
   int n = 42;
   auto args = fmt::make_wformat_args(n);