diff --git a/include/format b/include/format
index 167a5fba..633a731c 100644
--- a/include/format
+++ b/include/format
@@ -716,34 +716,36 @@ string vformat(string_view fmt, format_args args) {
 
 wstring vformat(wstring_view fmt, wformat_args args);
 
-template<FMT_CONCEPT(OutputIterator<const char&>) O, class... Args>
-  O format_to(O out, string_view fmt, const Args&... args) {
-    return vformat_to(out, fmt, {make_format_args<basic_format_context<O, char>>(args...)});
+template<class Out, class... Args>
+  Out format_to(Out out, string_view fmt, const Args&... args) {
+    using context = basic_format_context<Out, decltype(fmt)::value_type>;
+    return vformat_to(out, fmt, {make_format_args<context>(args...)});
   }
 
-template<FMT_CONCEPT(OutputIterator<const wchar_t&>) O, class... Args>
-  O format_to(O out, wstring_view fmt, const Args&... args) {
-    return vformat_to(out, fmt, {make_format_args<basic_format_context<O, wchar_t>>(args...)});
+template<class Out, class... Args>
+  Out format_to(Out out, wstring_view fmt, const Args&... args) {
+    using context = basic_format_context<Out, decltype(fmt)::value_type>;
+    return vformat_to(out, fmt, {make_format_args<context>(args...)});
   }
 
-template<FMT_CONCEPT(OutputIterator<const char&>) O>
-  O vformat_to(O out, string_view fmt, format_args_t<O, char> args) {
-    typedef fmt::output_range<O, char> range;
-    detail::format_handler<detail::arg_formatter<range>, char, basic_format_context<O, char>>
+template<class Out>
+  Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args) {
+    typedef fmt::output_range<Out, char> range;
+    detail::format_handler<detail::arg_formatter<range>, char, basic_format_context<Out, char>>
       h(range(out), fmt, args, {});
     fmt::internal::parse_format_string<false>(fmt::to_string_view(fmt), h);
     return h.context.out();
   }
 
-template<FMT_CONCEPT(OutputIterator<const wchar_t&>) O>
-  O vformat_to(O out, wstring_view fmt, format_args_t<O, wchar_t> args);
+template<class Out>
+  Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args);
 
-template<FMT_CONCEPT(OutputIterator<const char&>) O, class... Args>
-  format_to_n_result<O> format_to_n(O out, iter_difference_t<O> n,
-                                    string_view fmt, const Args&... args);
-template<FMT_CONCEPT(OutputIterator<const wchar_t&>) O, class... Args>
-  format_to_n_result<O> format_to_n(O out, iter_difference_t<O> n,
-                                    wstring_view fmt, const Args&... args);
+template<class Out, class... Args>
+  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
+                                      string_view fmt, const Args&... args);
+template<class Out, class... Args>
+  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
+                                      wstring_view fmt, const Args&... args);
 
 template<class... Args>
   size_t formatted_size(string_view fmt, const Args&... args);
@@ -764,11 +766,11 @@ template<size_t N> struct formatter<const charT[N], charT>
       : detail::formatter<std::basic_string_view<charT>, charT> {};
 
 template<class traits, class Allocator>
-  struct formatter<std::basic_string<charT, traits, Allocator>, charT>
+  struct formatter<basic_string<charT, traits, Allocator>, charT>
       : detail::formatter<std::basic_string_view<charT>, charT> {};
 
 template<class traits>
-  struct formatter<std::basic_string_view<charT, traits>, charT>
+  struct formatter<basic_string_view<charT, traits>, charT>
       : detail::formatter<std::basic_string_view<charT>, charT> {};
 
 template <> struct formatter<nullptr_t, charT> : detail::formatter<const void*, charT> {};
@@ -845,4 +847,50 @@ template <> struct formatter<long double, charT> : detail::formatter<long double
   };
 }
 
+inline void test0() {
+  using namespace std;
+  string s = format("{0}-{{", 8); // s == "8-{"
+}
+
+inline void test1() {
+  using namespace std;
+  string s0 = format("{} to {}",   "a", "b"); // OK: automatic indexing
+  string s1 = format("{1} to {0}", "a", "b"); // OK: manual indexing
+  string s2 = format("{0} to {}",  "a", "b"); // Error: mixing automatic and manual indexing
+  string s3 = format("{} to {1}",  "a", "b"); // Error: mixing automatic and manual indexing
+}
+
+inline void test2() {
+  using namespace std;
+  char c = 120;
+  string s0 = format("{:6}", 42);      // s0 == "    42"
+  string s1 = format("{:6}", 'x');     // s1 == "x     "
+  string s2 = format("{:*<6}", 'x');   // s2 == "x*****"
+  string s3 = format("{:*>6}", 'x');   // s3 == "*****x"
+  string s4 = format("{:*^6}", 'x');   // s4 == "**x***"
+  string s5 = format("{:=6}", 'x');    // Error: '=' with charT and no integer presentation type
+  string s6 = format("{:6d}", c);      // s6 == "   120"
+  string s7 = format("{:=+06d}", c);   // s7 == "+00120"
+  string s8 = format("{:0=#6x}", 0xa); // s8 == "0x000a"
+  string s9 = format("{:6}", true);    // s9 == "true  "
+}
+
+inline void test3() {
+  using namespace std;
+  double inf = numeric_limits<double>::infinity();
+  double nan = numeric_limits<double>::quiet_NaN();
+  string s0 = format("{0:} {0:+} {0:-} {0: }", 1);   // s0 == "1 +1 1  1"
+  string s1 = format("{0:} {0:+} {0:-} {0: }", -1);  // s1 == "-1 -1 -1 -1"
+  string s2 = format("{0:} {0:+} {0:-} {0: }", inf); // s2 == "inf +inf inf  inf"
+  string s3 = format("{0:} {0:+} {0:-} {0: }", nan); // s3 == "nan +nan nan  nan"
+}
+
+inline void test4() {
+  using namespace std;
+  string s0 = format("{}", 42);                      // s0 == "42"
+  string s1 = format("{0:b} {0:d} {0:o} {0:x}", 42); // s1 == "101010 42 52 2a"
+  string s2 = format("{0:#x} {0:#X}", 42);           // s2 == "0x2a 0X2A"
+  string s3 = format("{:n}", 1234);                  // s3 == "1,234" (depends on the locale)
+}
+
 #endif  // FMT_FORMAT_