From 9a4cc88426831595a29147cc32539c8141752267 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Sat, 20 Jun 2020 08:50:02 -0700
Subject: [PATCH] Add FMT_COMPILE support to format_to

---
 include/fmt/compile.h | 14 ++++++++++++--
 test/compile-test.cc  |  7 +++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/fmt/compile.h b/include/fmt/compile.h
index c85bf7c3..f5f08602 100644
--- a/include/fmt/compile.h
+++ b/include/fmt/compile.h
@@ -24,7 +24,9 @@ struct is_compiled_string : std::is_base_of<compiled_string, S> {};
 #define FMT_COMPILE(s) FMT_STRING_IMPL(s, fmt::detail::compiled_string)
 
 template <typename T, typename... Tail>
-const T& first(const T& value, const Tail&...) { return value; }
+const T& first(const T& value, const Tail&...) {
+  return value;
+}
 
 // Part of a compiled format string. It can be either literal text or a
 // replacement field.
@@ -556,7 +558,8 @@ std::basic_string<Char> format(const CompiledFormat& cf, const Args&... args) {
 
 template <typename S, typename... Args,
           FMT_ENABLE_IF(detail::is_compiled_string<S>::value)>
-FMT_INLINE std::basic_string<typename S::char_type> format(S, Args&&... args) {
+FMT_INLINE std::basic_string<typename S::char_type> format(const S&,
+                                                           Args&&... args) {
   constexpr basic_string_view<typename S::char_type> str = S();
   if (str.size() == 2 && str[0] == '{' && str[1] == '}')
     return fmt::to_string(detail::first(args...));
@@ -575,6 +578,13 @@ OutputIt format_to(OutputIt out, const CompiledFormat& cf,
                                          make_format_args<context>(args...));
 }
 
+template <typename OutputIt, typename S, typename... Args,
+          FMT_ENABLE_IF(detail::is_compiled_string<S>::value)>
+OutputIt format_to(OutputIt out, const S&, const Args&... args) {
+  constexpr auto compiled = compile<Args...>(S());
+  return format_to(out, compiled, args...);
+}
+
 template <
     typename OutputIt, typename CompiledFormat, typename... Args,
     FMT_ENABLE_IF(detail::is_output_iterator<OutputIt>::value&& std::is_base_of<
diff --git a/test/compile-test.cc b/test/compile-test.cc
index 83f8327c..a3f7322d 100644
--- a/test/compile-test.cc
+++ b/test/compile-test.cc
@@ -156,6 +156,13 @@ TEST(CompileTest, FormatDefault) {
   EXPECT_EQ("foo", fmt::format(FMT_COMPILE("{}"), std::string("foo")));
 }
 
+TEST(CompileTest, FormatTo) {
+  char buf[8];
+  auto end = fmt::format_to(buf, FMT_COMPILE("{}"), 42);
+  *end = '\0';
+  EXPECT_STREQ("42", buf);
+}
+
 TEST(CompileTest, TextAndArg) {
   EXPECT_EQ(">>>42<<<", fmt::format(FMT_COMPILE(">>>{}<<<"), 42));
 }