use Tim's approach

Author: huixie90

libcxx/include/__memory/shared_ptr.h

@@ -403,7 +403,7 @@ struct __shared_ptr_deleter_ctor_reqs {
                             __well_formed_deleter<_Dp, _Yp*>::value;
 };
 
-template <class _Dp, class _Tp>
+template <class _Dp>
 using __shared_ptr_nullptr_deleter_ctor_reqs = _And<is_move_constructible<_Dp>, __well_formed_deleter<_Dp, nullptr_t> >;
 
 #if defined(_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI)
@@ -414,6 +414,8 @@ using __shared_ptr_nullptr_deleter_ctor_reqs = _And<is_move_constructible<_Dp>,
 
 template <class _Tp>
 class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS shared_ptr {
+  struct __nullptr_sfinae_tag {};
+
 public:
 #if _LIBCPP_STD_VER >= 17
   typedef weak_ptr<_Tp> weak_type;
@@ -505,8 +507,12 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS shared_ptr {
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
   }
 
-  template <class _Dp, __enable_if_t<__shared_ptr_nullptr_deleter_ctor_reqs<_Dp, _Tp>::value, int> = 0 >
-  _LIBCPP_HIDE_FROM_ABI shared_ptr(nullptr_t __p, _Dp __d) : __ptr_(nullptr) {
+  template <class _Dp>
+  _LIBCPP_HIDE_FROM_ABI shared_ptr(
+      nullptr_t __p,
+      _Dp __d,
+      __enable_if_t<__shared_ptr_nullptr_deleter_ctor_reqs<_Dp>::value, __nullptr_sfinae_tag> = __nullptr_sfinae_tag())
+      : __ptr_(nullptr) {
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
     try {
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
@@ -525,8 +531,13 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS shared_ptr {
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
   }
 
-  template <class _Dp, class _Alloc, __enable_if_t<__shared_ptr_nullptr_deleter_ctor_reqs<_Dp, _Tp>::value, int> = 0 >
-  _LIBCPP_HIDE_FROM_ABI shared_ptr(nullptr_t __p, _Dp __d, _Alloc __a) : __ptr_(nullptr) {
+  template <class _Dp, class _Alloc>
+  _LIBCPP_HIDE_FROM_ABI shared_ptr(
+      nullptr_t __p,
+      _Dp __d,
+      _Alloc __a,
+      __enable_if_t<__shared_ptr_nullptr_deleter_ctor_reqs<_Dp>::value, __nullptr_sfinae_tag> = __nullptr_sfinae_tag())
+      : __ptr_(nullptr) {
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
     try {
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS