From b41890c1e50e952b79bc4a26ca5183d97b0c11a4 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Sun, 27 Mar 2022 07:10:15 -0700
Subject: [PATCH] Make arg_mapper SFINAE-friendly again

---
 include/fmt/core.h | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 4d83df79..0fa70208 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -1282,6 +1282,15 @@ inline auto format_as(std::byte b) -> unsigned char {
 }
 #endif
 
+template <typename T> struct has_format_as {
+  template <typename U, typename V = decltype(format_as(U())),
+            FMT_ENABLE_IF(std::is_enum<U>::value&& std::is_integral<V>::value)>
+  static auto check(U*) -> std::true_type;
+  static auto check(...) -> std::false_type;
+
+  enum { value = decltype(check(static_cast<T*>(nullptr)))::value };
+};
+
 // Maps formatting arguments to core types.
 // arg_mapper reports errors by returning unformattable instead of using
 // static_assert because it's used in the is_formattable trait.
@@ -1436,11 +1445,10 @@ template <typename Context> struct arg_mapper {
     return map(static_cast<underlying_t<T>>(val));
   }
 
-  template <typename T, typename U = decltype(format_as(T())),
-            FMT_ENABLE_IF(std::is_enum<T>::value&& std::is_integral<U>::value)>
+  template <typename T, FMT_ENABLE_IF(has_format_as<T>::value &&
+                                      !has_formatter<T, Context>::value)>
   FMT_CONSTEXPR FMT_INLINE auto map(const T& val)
-      -> decltype(std::declval<arg_mapper>().map(U())) {
-    static_assert(!has_formatter<T, Context>::value, "");
+      -> decltype(std::declval<arg_mapper>().map(format_as(T()))) {
     return map(format_as(val));
   }
 
@@ -1470,6 +1478,7 @@ template <typename Context> struct arg_mapper {
   template <typename T, typename U = remove_cvref_t<T>,
             FMT_ENABLE_IF(!is_string<U>::value && !is_char<U>::value &&
                           !std::is_array<U>::value &&
+                          !has_format_as<U>::value &&
                           (has_formatter<U, Context>::value ||
                            has_fallback_formatter<U, char_type>::value))>
   FMT_CONSTEXPR FMT_INLINE auto map(T&& val)