Fix format_to + FMT_STRING for wide character type

Fixes issue #3802

Author: Hans-Martin B. Jensen

Diff for: include/fmt/xchar.h

@@ -188,8 +188,8 @@ 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)>
-inline auto format_to(OutputIt out, const S& fmt, T&&... args) -> OutputIt {
-  return vformat_to(out, detail::to_string_view(fmt),
+inline auto format_to(OutputIt out, const S& format_str, T&&... args) -> OutputIt {
+  return vformat_to(out, fmt::basic_string_view<Char>(format_str),
                     fmt::make_format_args<buffered_context<Char>>(args...));
 }
 
@@ -215,7 +215,7 @@ template <typename OutputIt, typename Locale, typename S, typename... T,
 inline auto format_to(OutputIt out, const Locale& loc, const S& format_str,
                       T&&... args) ->
     typename std::enable_if<enable, OutputIt>::type {
-  return vformat_to(out, loc, detail::to_string_view(format_str),
+  return vformat_to(out, loc, fmt::basic_string_view<Char>(format_str),
                     fmt::make_format_args<buffered_context<Char>>(args...));
 }
 

Diff for: test/xchar-test.cc

@@ -93,6 +93,9 @@ TEST(xchar_test, compile_time_string) {
   EXPECT_EQ(fmt::format(fmt::wformat_string<int>(L"{}"), 42), L"42");
 #if defined(FMT_USE_STRING_VIEW) && FMT_CPLUSPLUS >= 201703L
   EXPECT_EQ(fmt::format(FMT_STRING(std::wstring_view(L"{}")), 42), L"42");
+  std::wstring ws;
+  fmt::format_to(std::back_inserter(ws), FMT_STRING(L"{}"), 42);
+  EXPECT_EQ(L"42", ws);
 #endif
 }