From 0e62e5dc7cd1132020e209023d919f8325d6d539 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Mon, 16 Sep 2024 19:48:02 -0700
Subject: [PATCH] Simplify has_formatter

---
 include/fmt/base.h | 23 ++++++++++-------------
 test/base-test.cc  |  6 +++---
 2 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 40637c56..80a31442 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -1108,26 +1108,23 @@ struct use_formatter
                     !use_format_as<T>::value> {};
 
 template <typename Char, typename T>
-constexpr auto has_const_formatter_impl(T*)
-    -> decltype(formatter<T, Char>().format(
-                    std::declval<const T&>(),
+constexpr auto has_formatter_impl(T*)
+    -> decltype(formatter<remove_const_t<T>, Char>().format(
+                    std::declval<T&>(),
                     std::declval<buffered_context<Char>&>()),
                 true) {
   return true;
 }
-template <typename Char> constexpr auto has_const_formatter_impl(...) -> bool {
+template <typename Char> constexpr auto has_formatter_impl(...) -> bool {
   return false;
 }
-template <typename T, typename Char>
-constexpr auto has_const_formatter() -> bool {
-  return has_const_formatter_impl<Char>(static_cast<T*>(nullptr));
+// T can be const-qualified to check if it is const-formattable.
+template <typename T, typename Char> constexpr auto has_formatter() -> bool {
+  return has_formatter_impl<Char>(static_cast<T*>(nullptr));
 }
 
-template <typename T, typename Char, typename U = remove_const_t<T>>
-struct formattable
-    : bool_constant<has_const_formatter<U, Char>() ||
-                    (std::is_constructible<formatter<U, Char>>::value &&
-                     !std::is_const<T>::value)> {};
+template <typename T, typename Char>
+struct formattable : bool_constant<has_formatter<T, Char>()> {};
 
 // Maps formatting argument types to natively supported types or user-defined
 // types with formatters. Returns void on errors to be SFINAE-friendly.
@@ -2223,7 +2220,7 @@ template <typename Context> class value {
     auto f = Formatter();
     parse_ctx.advance_to(f.parse(parse_ctx));
     using qualified_type =
-        conditional_t<has_const_formatter<T, char_type>(), const T, T>;
+        conditional_t<has_formatter<const T, char_type>(), const T, T>;
     // format must be const for compatibility with std::format and compilation.
     const auto& cf = f;
     ctx.advance_to(cf.format(*static_cast<qualified_type*>(arg), ctx));
diff --git a/test/base-test.cc b/test/base-test.cc
index 3189858a..cc0901a4 100644
--- a/test/base-test.cc
+++ b/test/base-test.cc
@@ -784,10 +784,10 @@ TEST(base_test, format_explicitly_convertible_to_std_string_view) {
 #  endif
 #endif
 
-TEST(base_test, has_const_formatter) {
-  EXPECT_TRUE((fmt::detail::has_const_formatter<const_formattable, char>()));
+TEST(base_test, has_formatter) {
+  EXPECT_TRUE((fmt::detail::has_formatter<const const_formattable, char>()));
   EXPECT_FALSE(
-      (fmt::detail::has_const_formatter<nonconst_formattable, char>()));
+      (fmt::detail::has_formatter<const nonconst_formattable, char>()));
 }
 
 TEST(base_test, format_nonconst) {