Detect types convertible to unformattable pointers

include/fmt/core.h

@@ -1370,8 +1370,11 @@ template <typename Context> struct arg_mapper {
 
   // We use SFINAE instead of a const T* parameter to avoid conflicting with
   // the C array overload.
-  template <typename T, FMT_ENABLE_IF(std::is_pointer<T>::value)>
-  FMT_CONSTEXPR auto map(T) -> unformattable_pointer {
+  template <
+      typename T,
+      FMT_ENABLE_IF(std::is_convertible<const T&, const void*>::value &&
+                    !std::is_convertible<const T&, const char_type*>::value)>
+  FMT_CONSTEXPR auto map(const T&) -> unformattable_pointer {
     return {};
   }
 

test/core-test.cc

@@ -733,6 +733,10 @@ template <> struct formatter<nonconst_formattable> {
 };
 FMT_END_NAMESPACE
 
+struct convertible_to_pointer {
+  operator const int*() const { return nullptr; }
+};
+
 TEST(core_test, is_formattable) {
   static_assert(fmt::is_formattable<signed char*>::value, "");
   static_assert(fmt::is_formattable<unsigned char*>::value, "");
@@ -760,6 +764,8 @@ TEST(core_test, is_formattable) {
   static_assert(!fmt::is_formattable<const nonconst_formattable&>::value, "");
 #endif
 
+  static_assert(!fmt::is_formattable<convertible_to_pointer>::value, "");
+
   static_assert(!fmt::is_formattable<signed char*, wchar_t>::value, "");
   static_assert(!fmt::is_formattable<unsigned char*, wchar_t>::value, "");
   static_assert(!fmt::is_formattable<const signed char*, wchar_t>::value, "");