Support fmt::runtime with wchar_t in fmt::format_to_n

Author: sunmy2019

include/fmt/base.h

@@ -906,6 +906,9 @@ class locale_ref {
 
 FMT_END_EXPORT
 
+// forward declaration
+template <typename Char> class runtime_format_string;
+
 namespace detail {
 
 // Specifies if `T` is a code unit type.
@@ -935,6 +938,19 @@ constexpr auto to_string_view(basic_string_view<Char> s)
     -> basic_string_view<Char> {
   return s;
 }
+template <typename Char>
+constexpr auto to_string_view(runtime_format_string<Char> s)
+    -> basic_string_view<Char> {
+  return s.str;
+}
+// forward declaration of internal compile-time string type base
+class compile_string;
+template <typename Char, typename S,
+          FMT_ENABLE_IF(std::is_base_of<detail::compile_string, S>::value&&
+                            std::is_same<typename S::char_type, Char>::value)>
+constexpr auto to_string_view(const S& s) -> basic_string_view<Char> {
+  return basic_string_view<Char>(s);
+}
 
 template <typename T, typename Enable = void>
 struct has_to_string_view : std::false_type {};

include/fmt/xchar.h

@@ -285,7 +285,7 @@ template <typename OutputIt, typename S, typename... T,
                             detail::is_exotic_char<Char>::value)>
 inline auto format_to_n(OutputIt out, size_t n, const S& fmt, T&&... args)
     -> format_to_n_result<OutputIt> {
-  return vformat_to_n(out, n, fmt::basic_string_view<Char>(fmt),
+  return vformat_to_n(out, n, detail::to_string_view<Char>(fmt),
                       fmt::make_format_args<buffered_context<Char>>(args...));
 }
 

test/xchar-test.cc

@@ -146,6 +146,26 @@ TEST(format_test, wide_format_to_n) {
   EXPECT_EQ(L"BC x", fmt::wstring_view(buffer, 4));
 }
 
+TEST(format_test, wide_format_to_n_runtime) {
+  wchar_t buffer[4];
+  buffer[3] = L'x';
+  auto result = fmt::format_to_n(buffer, 3, fmt::runtime(L"{}"), 12345);
+  EXPECT_EQ(5u, result.size);
+  EXPECT_EQ(buffer + 3, result.out);
+  EXPECT_EQ(L"123x", fmt::wstring_view(buffer, 4));
+  buffer[0] = L'x';
+  buffer[1] = L'x';
+  buffer[2] = L'x';
+  result = fmt::format_to_n(buffer, 3, fmt::runtime(L"{}"), L'A');
+  EXPECT_EQ(1u, result.size);
+  EXPECT_EQ(buffer + 1, result.out);
+  EXPECT_EQ(L"Axxx", fmt::wstring_view(buffer, 4));
+  result = fmt::format_to_n(buffer, 3, fmt::runtime(L"{}{} "), L'B', L'C');
+  EXPECT_EQ(3u, result.size);
+  EXPECT_EQ(buffer + 3, result.out);
+  EXPECT_EQ(L"BC x", fmt::wstring_view(buffer, 4));
+}
+
 TEST(xchar_test, named_arg_udl) {
   using namespace fmt::literals;
   auto udl_a =