From 5de0bc1d4f2e7a750f050ca2e80ab95586f96d8e Mon Sep 17 00:00:00 2001
From: Alexey Ochapov <alexez@alexez.com>
Date: Tue, 8 Dec 2020 02:53:11 +0300
Subject: [PATCH] Add UDL as replacement for FMT_COMPILE (#2043)

---
 include/fmt/compile.h | 38 ++++++++++++++++++++++++++++++++++++++
 test/compile-test.cc  |  8 ++++++++
 2 files changed, 46 insertions(+)

diff --git a/include/fmt/compile.h b/include/fmt/compile.h
index affd793e..65aeb445 100644
--- a/include/fmt/compile.h
+++ b/include/fmt/compile.h
@@ -13,6 +13,15 @@
 
 #include "format.h"
 
+#ifndef FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+#  if defined(__cpp_nontype_template_parameter_class) && \
+      (!FMT_GCC_VERSION || FMT_GCC_VERSION >= 903)
+#    define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 1
+#  else
+#    define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 0
+#  endif
+#endif
+
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
@@ -37,6 +46,24 @@ struct is_compiled_string : std::is_base_of<compiled_string, S> {};
  */
 #define FMT_COMPILE(s) FMT_STRING_IMPL(s, fmt::detail::compiled_string)
 
+#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+template <typename Char, size_t N> struct fixed_string {
+  constexpr fixed_string(const Char (&str)[N]) {
+    copy_str<Char, const Char*, Char*>(static_cast<const Char*>(str), str + N,
+                                       data);
+  }
+  Char data[N]{};
+};
+
+template <typename Char, size_t N, fixed_string<Char, N> Str>
+struct udl_compiled_string : compiled_string {
+  using char_type = Char;
+  constexpr operator basic_string_view<char_type>() const {
+    return {Str.data, N - 1};
+  }
+};
+#endif
+
 template <typename T, typename... Tail>
 const T& first(const T& value, const Tail&...) {
   return value;
@@ -698,6 +725,17 @@ size_t formatted_size(const CompiledFormat& cf, const Args&... args) {
   return format_to(detail::counting_iterator(), cf, args...).count();
 }
 
+#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+inline namespace literals {
+template <detail::fixed_string Str>
+constexpr detail::udl_compiled_string<remove_cvref_t<decltype(Str.data[0])>,
+                                      sizeof(Str.data), Str>
+operator""_cf() {
+  return {};
+}
+}  // namespace literals
+#endif
+
 FMT_END_NAMESPACE
 
 #endif  // FMT_COMPILE_H_
diff --git a/test/compile-test.cc b/test/compile-test.cc
index 479de061..4038ac58 100644
--- a/test/compile-test.cc
+++ b/test/compile-test.cc
@@ -179,6 +179,14 @@ TEST(CompileTest, Empty) {
 }
 #endif
 
+#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+TEST(CompileTest, CompileFormatStringLiteral) {
+  using namespace fmt::literals;
+  EXPECT_EQ("", fmt::format(""_cf));
+  EXPECT_EQ("42", fmt::format("{}"_cf, 42));
+}
+#endif
+
 #if __cplusplus >= 202002L
 template <size_t max_string_length> struct test_string {
   template <typename T> constexpr bool operator==(const T& rhs) const noexcept {