diff --git a/include/fmt/color.h b/include/fmt/color.h
index 49407e3c..9e1bc536 100644
--- a/include/fmt/color.h
+++ b/include/fmt/color.h
@@ -483,7 +483,7 @@ template <typename S, typename Char = char_t<S>>
 void vprint(std::FILE* f, const text_style& ts, const S& format,
             basic_format_args<buffer_context<type_identity_t<Char>>> args) {
   basic_memory_buffer<Char> buf;
-  detail::vformat_to(buf, ts, to_string_view(format), args);
+  detail::vformat_to(buf, ts, detail::to_string_view(format), args);
   if (detail::is_utf8()) {
     detail::print(f, basic_string_view<Char>(buf.begin(), buf.size()));
   } else {
@@ -533,7 +533,7 @@ inline std::basic_string<Char> vformat(
     const text_style& ts, const S& format_str,
     basic_format_args<buffer_context<type_identity_t<Char>>> args) {
   basic_memory_buffer<Char> buf;
-  detail::vformat_to(buf, ts, to_string_view(format_str), args);
+  detail::vformat_to(buf, ts, detail::to_string_view(format_str), args);
   return fmt::to_string(buf);
 }
 
@@ -552,7 +552,7 @@ inline std::basic_string<Char> vformat(
 template <typename S, typename... Args, typename Char = char_t<S>>
 inline std::basic_string<Char> format(const text_style& ts, const S& format_str,
                                       const Args&... args) {
-  return fmt::vformat(ts, to_string_view(format_str),
+  return fmt::vformat(ts, detail::to_string_view(format_str),
                       fmt::make_format_args<buffer_context<Char>>(args...));
 }
 
@@ -587,7 +587,7 @@ template <typename OutputIt, typename S, typename... Args,
 inline auto format_to(OutputIt out, const text_style& ts, const S& format_str,
                       Args&&... args) ->
     typename std::enable_if<enable, OutputIt>::type {
-  return vformat_to(out, ts, to_string_view(format_str),
+  return vformat_to(out, ts, detail::to_string_view(format_str),
                     fmt::make_format_args<buffer_context<char_t<S>>>(args...));
 }
 
diff --git a/include/fmt/core.h b/include/fmt/core.h
index 762155c4..c62ffdf6 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -493,6 +493,15 @@ using string_view = basic_string_view<char>;
 template <typename T> struct is_char : std::false_type {};
 template <> struct is_char<char> : std::true_type {};
 
+// A base class for compile-time strings. It is defined in the fmt namespace to
+// make formatting functions visible via ADL, e.g. format(FMT_STRING("{}"), 42).
+struct compile_string {};
+
+template <typename S>
+struct is_compile_string : std::is_base_of<compile_string, S> {};
+
+FMT_BEGIN_DETAIL_NAMESPACE
+
 // Returns a string view of `s`.
 template <typename Char, FMT_ENABLE_IF(is_char<Char>::value)>
 FMT_INLINE auto to_string_view(const Char* s) -> basic_string_view<Char> {
@@ -509,29 +518,17 @@ constexpr auto to_string_view(basic_string_view<Char> s)
   return s;
 }
 template <typename Char,
-          FMT_ENABLE_IF(!std::is_empty<detail::std_string_view<Char>>::value)>
-inline auto to_string_view(detail::std_string_view<Char> s)
+          FMT_ENABLE_IF(!std::is_empty<std_string_view<Char>>::value)>
+inline auto to_string_view(std_string_view<Char> s)
     -> basic_string_view<Char> {
   return s;
 }
-
-// A base class for compile-time strings. It is defined in the fmt namespace to
-// make formatting functions visible via ADL, e.g. format(FMT_STRING("{}"), 42).
-struct compile_string {};
-
-template <typename S>
-struct is_compile_string : std::is_base_of<compile_string, S> {};
-
 template <typename S, FMT_ENABLE_IF(is_compile_string<S>::value)>
 constexpr auto to_string_view(const S& s)
     -> basic_string_view<typename S::char_type> {
   return basic_string_view<typename S::char_type>(s);
 }
-
-FMT_BEGIN_DETAIL_NAMESPACE
-
 void to_string_view(...);
-using fmt::to_string_view;
 
 // Specifies whether S is a string type convertible to fmt::basic_string_view.
 // It should be a constexpr function but MSVC 2017 fails to compile it in
diff --git a/include/fmt/printf.h b/include/fmt/printf.h
index abb67ed0..70a592dc 100644
--- a/include/fmt/printf.h
+++ b/include/fmt/printf.h
@@ -560,7 +560,7 @@ inline auto vsprintf(
     basic_format_args<basic_printf_context_t<type_identity_t<Char>>> args)
     -> std::basic_string<Char> {
   basic_memory_buffer<Char> buffer;
-  vprintf(buffer, to_string_view(fmt), args);
+  vprintf(buffer, detail::to_string_view(fmt), args);
   return to_string(buffer);
 }
 
@@ -577,7 +577,8 @@ template <typename S, typename... T,
           typename Char = enable_if_t<detail::is_string<S>::value, char_t<S>>>
 inline auto sprintf(const S& fmt, const T&... args) -> std::basic_string<Char> {
   using context = basic_printf_context_t<Char>;
-  return vsprintf(to_string_view(fmt), fmt::make_format_args<context>(args...));
+  return vsprintf(detail::to_string_view(fmt),
+                  fmt::make_format_args<context>(args...));
 }
 
 template <typename S, typename Char = char_t<S>>
@@ -586,7 +587,7 @@ inline auto vfprintf(
     basic_format_args<basic_printf_context_t<type_identity_t<Char>>> args)
     -> int {
   basic_memory_buffer<Char> buffer;
-  vprintf(buffer, to_string_view(fmt), args);
+  vprintf(buffer, detail::to_string_view(fmt), args);
   size_t size = buffer.size();
   return std::fwrite(buffer.data(), sizeof(Char), size, f) < size
              ? -1
@@ -605,7 +606,7 @@ inline auto vfprintf(
 template <typename S, typename... T, typename Char = char_t<S>>
 inline auto fprintf(std::FILE* f, const S& fmt, const T&... args) -> int {
   using context = basic_printf_context_t<Char>;
-  return vfprintf(f, to_string_view(fmt),
+  return vfprintf(f, detail::to_string_view(fmt),
                   fmt::make_format_args<context>(args...));
 }
 
@@ -614,7 +615,7 @@ inline auto vprintf(
     const S& fmt,
     basic_format_args<basic_printf_context_t<type_identity_t<Char>>> args)
     -> int {
-  return vfprintf(stdout, to_string_view(fmt), args);
+  return vfprintf(stdout, detail::to_string_view(fmt), args);
 }
 
 /**
@@ -629,7 +630,7 @@ inline auto vprintf(
 template <typename S, typename... T, FMT_ENABLE_IF(detail::is_string<S>::value)>
 inline auto printf(const S& fmt, const T&... args) -> int {
   return vprintf(
-      to_string_view(fmt),
+      detail::to_string_view(fmt),
       fmt::make_format_args<basic_printf_context_t<char_t<S>>>(args...));
 }
 
diff --git a/include/fmt/xchar.h b/include/fmt/xchar.h
index 6e3e76de..ea34917f 100644
--- a/include/fmt/xchar.h
+++ b/include/fmt/xchar.h
@@ -92,7 +92,7 @@ auto vformat(basic_string_view<Char> format_str,
 template <typename S, typename... Args, typename Char = char_t<S>,
           FMT_ENABLE_IF(!std::is_same<Char, char>::value)>
 auto format(const S& format_str, Args&&... args) -> std::basic_string<Char> {
-  return vformat(to_string_view(format_str),
+  return vformat(detail::to_string_view(format_str),
                  fmt::make_format_args<buffer_context<Char>>(args...));
 }
 
@@ -103,7 +103,7 @@ inline auto vformat(
     const Locale& loc, const S& format_str,
     basic_format_args<buffer_context<type_identity_t<Char>>> args)
     -> std::basic_string<Char> {
-  return detail::vformat(loc, to_string_view(format_str), args);
+  return detail::vformat(loc, detail::to_string_view(format_str), args);
 }
 
 template <typename Locale, typename S, typename... Args,
@@ -112,7 +112,7 @@ template <typename Locale, typename S, typename... Args,
                             detail::is_exotic_char<Char>::value)>
 inline auto format(const Locale& loc, const S& format_str, Args&&... args)
     -> std::basic_string<Char> {
-  return detail::vformat(loc, to_string_view(format_str),
+  return detail::vformat(loc, detail::to_string_view(format_str),
                          fmt::make_format_args<buffer_context<Char>>(args...));
 }
 
@@ -123,7 +123,7 @@ auto vformat_to(OutputIt out, const S& format_str,
                 basic_format_args<buffer_context<type_identity_t<Char>>> args)
     -> OutputIt {
   auto&& buf = detail::get_buffer<Char>(out);
-  detail::vformat_to(buf, to_string_view(format_str), args);
+  detail::vformat_to(buf, detail::to_string_view(format_str), args);
   return detail::get_iterator(buf);
 }
 
@@ -132,7 +132,7 @@ template <typename OutputIt, typename S, typename... Args,
           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, Args&&... args) -> OutputIt {
-  return vformat_to(out, to_string_view(fmt),
+  return vformat_to(out, detail::to_string_view(fmt),
                     fmt::make_format_args<buffer_context<Char>>(args...));
 }
 
@@ -145,7 +145,8 @@ inline auto vformat_to(
     OutputIt out, const Locale& loc, const S& format_str,
     basic_format_args<buffer_context<type_identity_t<Char>>> args) -> OutputIt {
   auto&& buf = detail::get_buffer<Char>(out);
-  vformat_to(buf, to_string_view(format_str), args, detail::locale_ref(loc));
+  vformat_to(buf, detail::to_string_view(format_str), args,
+             detail::locale_ref(loc));
   return detail::get_iterator(buf);
 }
 
@@ -180,7 +181,7 @@ template <typename OutputIt, typename S, typename... Args,
                             detail::is_exotic_char<Char>::value)>
 inline auto format_to_n(OutputIt out, size_t n, const S& fmt,
                         const Args&... args) -> format_to_n_result<OutputIt> {
-  return vformat_to_n(out, n, to_string_view(fmt),
+  return vformat_to_n(out, n, detail::to_string_view(fmt),
                       fmt::make_format_args<buffer_context<Char>>(args...));
 }
 
@@ -188,7 +189,7 @@ template <typename S, typename... Args, typename Char = char_t<S>,
           FMT_ENABLE_IF(detail::is_exotic_char<Char>::value)>
 inline auto formatted_size(const S& fmt, Args&&... args) -> size_t {
   detail::counting_buffer<Char> buf;
-  detail::vformat_to(buf, to_string_view(fmt),
+  detail::vformat_to(buf, detail::to_string_view(fmt),
                      fmt::make_format_args<buffer_context<Char>>(args...));
   return buf.count();
 }
diff --git a/test/xchar-test.cc b/test/xchar-test.cc
index 7db35913..498ff651 100644
--- a/test/xchar-test.cc
+++ b/test/xchar-test.cc
@@ -240,28 +240,6 @@ TEST(xchar_test, sign_not_truncated) {
   EXPECT_THROW(fmt::format(format_str, 42), fmt::format_error);
 }
 
-namespace fake_qt {
-class QString {
- public:
-  QString(const wchar_t* s) : s_(s) {}
-  const wchar_t* utf16() const noexcept { return s_.data(); }
-  int size() const noexcept { return static_cast<int>(s_.size()); }
-
- private:
-  std::wstring s_;
-};
-
-fmt::basic_string_view<wchar_t> to_string_view(const QString& s) noexcept {
-  return {s.utf16(), static_cast<size_t>(s.size())};
-}
-}  // namespace fake_qt
-
-TEST(format_test, format_foreign_strings) {
-  using fake_qt::QString;
-  EXPECT_EQ(fmt::format(QString(L"{}"), 42), L"42");
-  EXPECT_EQ(fmt::format(QString(L"{}"), QString(L"42")), L"42");
-}
-
 TEST(xchar_test, chrono) {
   auto tm = std::tm();
   tm.tm_year = 116;