add: [imp] tag_invoke for fmt

include/libimp/fmt.h

@@ -17,6 +17,7 @@
 #include "libimp/span.h"
 #include "libimp/detect_plat.h"
 #include "libimp/export.h"
+#include "libimp/generic.h"
 
 LIBIMP_NAMESPACE_BEG_
 
@@ -34,17 +35,31 @@ auto spec(char const (&fstr)[N]) noexcept {
   };
 }
 
+namespace detail {
+
+struct fmt_to_string_t {
+  template <typename T>
+  std::string operator()(T &&arg) const {
+    return ::LIBIMP::tag_invoke(fmt_to_string_t{}, std::forward<T>(arg));
+  }
+};
+
+} // namespace detail
+
+constexpr detail::fmt_to_string_t fmt_to_string {};
+
 template <typename... A>
 std::string fmt(A &&...args) {
   std::string joined;
   LIBIMP_UNUSED auto unfold = {
-    joined.append(to_string(std::forward<A>(args)))...
+    joined.append(fmt_to_string(std::forward<A>(args)))...
   };
   return joined;
 }
 
 /// @brief Return the string directly.
-LIBIMP_EXPORT std::string const &to_string(std::string const &a) noexcept;
+LIBIMP_EXPORT std::string to_string(std::string const &a) noexcept;
+LIBIMP_EXPORT std::string to_string(std::string &&a) noexcept;
 LIBIMP_EXPORT std::string to_string(std::string const &a, span<char const> fstr) noexcept;
 
 /// @brief Character to string conversion.
@@ -92,4 +107,16 @@ LIBIMP_EXPORT std::string to_string(std::chrono::time_point<Clock, Duration> con
   return detail::time_to_string(std::chrono::system_clock::to_time_t(a), fstr);
 }
 
+/**
+ * @brief Predefined fmt_to_string method
+ */
+namespace detail {
+
+template <typename T>
+auto tag_invoke(fmt_to_string_t, T &&arg) noexcept
+  -> decltype(::LIBIMP::to_string(std::forward<T>(arg))) {
+  return ::LIBIMP::to_string(std::forward<T>(arg));
+}
+
+} // namespace detail
 LIBIMP_NAMESPACE_END_

include/libimp/generic.h

@@ -6,6 +6,8 @@
  */
 #pragma once
 
+#include <utility>
+
 #include "libimp/def.h"
 
 LIBIMP_NAMESPACE_BEG_
@@ -17,4 +19,25 @@ LIBIMP_NAMESPACE_BEG_
 template <typename...>
 using void_t = void;
 
+/**
+ * @brief A general pattern for supporting customisable functions
+ * @see https://www.open-std.org/jtc1/sc22/WG21/docs/papers/2019/p1895r0.pdf
+*/
+namespace detail {
+
+void tag_invoke();
+
+struct tag_invoke_t {
+  template <typename T, typename... A>
+  constexpr auto operator()(T tag, A &&...args) const
+    noexcept(noexcept(tag_invoke(std::forward<T>(tag), std::forward<A>(args)...)))
+          -> decltype(tag_invoke(std::forward<T>(tag), std::forward<A>(args)...)) {
+    return tag_invoke(std::forward<T>(tag), std::forward<A>(args)...);
+  }
+};
+
+} // namespace detail
+
+constexpr detail::tag_invoke_t tag_invoke {};
+
 LIBIMP_NAMESPACE_END_

include/libimp/log.h

@@ -17,6 +17,7 @@
 #include "libimp/detect_plat.h"
 #include "libimp/export.h"
 #include "libimp/enum_cast.h"
+#include "libimp/fmt.h"
 
 LIBIMP_NAMESPACE_BEG_
 namespace log {
@@ -39,6 +40,14 @@ struct context {
 
 LIBIMP_EXPORT std::string to_string(context &&) noexcept;
 
+/**
+ * @brief Custom defined fmt_to_string method for imp::fmt
+ */
+template <typename T>
+std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), context &&arg) noexcept {
+  return ::LIBIMP::log::to_string(std::move(arg));
+}
+
 } // namespace log
 
 namespace detail_log {

src/libimp/fmt.cpp

@@ -98,10 +98,14 @@ std::string sprintf(F fop, span<char const> fstr, span<char const> s, A a) noexc
 
 } // namespace
 
-std::string const &to_string(std::string const &a) noexcept {
+std::string to_string(std::string const &a) noexcept {
   return a;
 }
 
+std::string to_string(std::string &&a) noexcept {
+  return std::move(a);
+}
+
 std::string to_string(std::string const &a, span<char const> fstr) noexcept {
   if (a.empty()) return {};
   return ::LIBIMP::sprintf(fmt_of, fstr, "s", a.c_str());