Simplify has_formatter

fmtlib · Sep 17, 2024 · e20407f · e20407f
1 parent 7ce0139
commit e20407f
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Showing 2 changed files with 16 additions and 23 deletions.
diff --git a/include/fmt/base.h b/include/fmt/base.h
@@ -1107,27 +1107,20 @@ struct use_formatter
                     !has_to_string_view<T>::value && !is_named_arg<T>::value &&
                     !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&>(),
-                    std::declval<buffered_context<Char>&>()),
-                true) {
-  return true;
-}
-template <typename Char> constexpr auto has_const_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));
+template <typename Char, typename T,
+          FMT_ENABLE_IF(sizeof(formatter<remove_const_t<T>, Char>().format(
+                            std::declval<T&>(),
+                            std::declval<buffered_context<Char>&>())) != 0)>
+auto has_formatter_impl(T*) -> std::true_type;
+template <typename> auto has_formatter_impl(...) -> std::false_type;
+
+// T can be const-qualified to check if it is const-formattable.
+template <typename T, typename Char> constexpr auto has_formatter() -> bool {
+  return decltype(has_formatter_impl<Char>(static_cast<T*>(nullptr)))::value;
 }
 
-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 +2216,7 @@ 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
@@ -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) {