大家一般都是先关心参数，然后最后再看返回的是什么类型。

在这里把返回类型后置，可能就是一种逻辑上的体现吧

fmt的一个函数。

\fmt\core.h

这个函数的意义，应该就是用变长参数初始化成一个format_arg_store类型的变量，并返回。

\fmt\bundled\core.h

/**\rstConstructs a `~fmt::format_arg_store` object that contains references toarguments and can be implicitly converted to `~fmt::format_args`. `Context`can be omitted in which case it defaults to `~fmt::context`.See `~fmt::arg` for lifetime considerations.\endrst*/
template <typename Context = format_context, typename... Args>
constexpr auto make_format_args(const Args&... args)-> format_arg_store<Context, Args...> {return {args...};
}

但这个怎么回事，传进double，返回的还是double：

  FMT_CONSTEXPR FMT_INLINE auto map(double val) -> double { return val; }

可能是因为重载其他map模板函数，比如：

 template <typename T, FMT_ENABLE_IF(is_char<T>::value)>FMT_CONSTEXPR FMT_INLINE auto map(T val) -> char_type {static_assert(std::is_same<T, char>::value || std::is_same<T, char_type>::value,"mixing character types is disallowed");return val;}注意，这个char_type是通过模板参数而来的：
using char_type = typename Context::char_type;

\fmt\bundled\core.h

// Maps formatting arguments to core types.
template <typename Context> struct arg_mapper 
{using char_type = typename Context::char_type;FMT_CONSTEXPR FMT_INLINE auto map(signed char val) -> int { return val; }FMT_CONSTEXPR FMT_INLINE auto map(unsigned char val) -> unsigned {return val;}FMT_CONSTEXPR FMT_INLINE auto map(short val) -> int { return val; }FMT_CONSTEXPR FMT_INLINE auto map(unsigned short val) -> unsigned {return val;}FMT_CONSTEXPR FMT_INLINE auto map(int val) -> int { return val; }FMT_CONSTEXPR FMT_INLINE auto map(unsigned val) -> unsigned { return val; }FMT_CONSTEXPR FMT_INLINE auto map(long val) -> long_type { return val; }FMT_CONSTEXPR FMT_INLINE auto map(unsigned long val) -> ulong_type {return val;}FMT_CONSTEXPR FMT_INLINE auto map(long long val) -> long long { return val; }FMT_CONSTEXPR FMT_INLINE auto map(unsigned long long val)-> unsigned long long {return val;}FMT_CONSTEXPR FMT_INLINE auto map(int128_t val) -> int128_t { return val; }FMT_CONSTEXPR FMT_INLINE auto map(uint128_t val) -> uint128_t { return val; }FMT_CONSTEXPR FMT_INLINE auto map(bool val) -> bool { return val; }template <typename T, FMT_ENABLE_IF(is_char<T>::value)>FMT_CONSTEXPR FMT_INLINE auto map(T val) -> char_type {static_assert(std::is_same<T, char>::value || std::is_same<T, char_type>::value,"mixing character types is disallowed");return val;}FMT_CONSTEXPR FMT_INLINE auto map(float val) -> float { return val; }FMT_CONSTEXPR FMT_INLINE auto map(double val) -> double { return val; }FMT_CONSTEXPR FMT_INLINE auto map(long double val) -> long double {return val;}FMT_CONSTEXPR FMT_INLINE auto map(char_type* val) -> const char_type* {return val;}FMT_CONSTEXPR FMT_INLINE auto map(const char_type* val) -> const char_type* {return val;}template <typename T, FMT_ENABLE_IF(is_string<T>::value)>FMT_CONSTEXPR FMT_INLINE auto map(const T& val)-> basic_string_view<char_type> {static_assert(std::is_same<char_type, char_t<T>>::value,"mixing character types is disallowed");return to_string_view(val);}template <typename T,FMT_ENABLE_IF(std::is_constructible<basic_string_view<char_type>, T>::value &&!is_string<T>::value && !has_formatter<T, Context>::value &&!has_fallback_formatter<T, char_type>::value)>FMT_CONSTEXPR FMT_INLINE auto map(const T& val)-> basic_string_view<char_type> {return basic_string_view<char_type>(val);}template <typename T,FMT_ENABLE_IF(std::is_constructible<std_string_view<char_type>, T>::value &&!std::is_constructible<basic_string_view<char_type>, T>::value &&!is_string<T>::value && !has_formatter<T, Context>::value &&!has_fallback_formatter<T, char_type>::value)>FMT_CONSTEXPR FMT_INLINE auto map(const T& val)-> basic_string_view<char_type> {return std_string_view<char_type>(val);}FMT_CONSTEXPR FMT_INLINE auto map(const signed char* val) -> const char* {static_assert(std::is_same<char_type, char>::value, "invalid string type");return reinterpret_cast<const char*>(val);}FMT_CONSTEXPR FMT_INLINE auto map(const unsigned char* val) -> const char* {static_assert(std::is_same<char_type, char>::value, "invalid string type");return reinterpret_cast<const char*>(val);}FMT_CONSTEXPR FMT_INLINE auto map(signed char* val) -> const char* {const auto* const_val = val;return map(const_val);}FMT_CONSTEXPR FMT_INLINE auto map(unsigned char* val) -> const char* {const auto* const_val = val;return map(const_val);}FMT_CONSTEXPR FMT_INLINE auto map(void* val) -> const void* { return val; }FMT_CONSTEXPR FMT_INLINE auto map(const void* val) -> const void* {return val;}FMT_CONSTEXPR FMT_INLINE auto map(std::nullptr_t val) -> const void* {return val;}// We use SFINAE instead of a const T* parameter to avoid conflicting with// the C array overload.template <typename T>FMT_CONSTEXPR auto map(T) -> enable_if_t<std::is_pointer<T>::value, int> {// Formatting of arbitrary pointers is disallowed. If you want to output// a pointer cast it to "void *" or "const void *". In particular, this// forbids formatting of "[const] volatile char *" which is printed as bool// by iostreams.static_assert(!sizeof(T), "formatting of non-void pointers is disallowed");return 0;}template <typename T, std::size_t N>FMT_CONSTEXPR FMT_INLINE auto map(const T (&values)[N]) -> const T (&)[N] {return values;}template <typename T,FMT_ENABLE_IF(std::is_enum<T>::value &&!has_formatter<T, Context>::value &&!has_fallback_formatter<T, char_type>::value)>FMT_CONSTEXPR FMT_INLINE auto map(const T& val)-> decltype(std::declval<arg_mapper>().map(static_cast<typename std::underlying_type<T>::type>(val))) {return map(static_cast<typename std::underlying_type<T>::type>(val));}template <typename T,FMT_ENABLE_IF(!is_string<T>::value && !is_char<T>::value &&(has_formatter<T, Context>::value ||has_fallback_formatter<T, char_type>::value))>FMT_CONSTEXPR FMT_INLINE auto map(const T& val) -> const T& {return val;}template <typename T, FMT_ENABLE_IF(is_named_arg<T>::value)>FMT_CONSTEXPR FMT_INLINE auto map(const T& named_arg)-> decltype(std::declval<arg_mapper>().map(named_arg.value)) {return map(named_arg.value);}auto map(...) -> unformattable { return {}; }
}