Systematically replacing detail::enable_if_t<...smart_holder...> with typename std::enable_if<...smart_holder...>::type. Attempt to work around MSVC 2015 issues, to be tested via GitHub CI. The idea for this change originates from this comment: #1616 (comment)

Author: rwgk

include/pybind11/cast.h

@@ -2459,7 +2459,7 @@ struct is_smart_holder_type_caster : std::false_type {};
 template <typename T>
 struct is_smart_holder_type_caster<
     T,
-    enable_if_t<type_caster<T>::is_smart_holder_type_caster::value, void>> : std::true_type {};
+    typename std::enable_if<type_caster<T>::is_smart_holder_type_caster::value, void>::type> : std::true_type {};
 
 template <typename T>
 inline bool check_is_smart_holder_type_caster() {

include/pybind11/detail/init.h

@@ -132,7 +132,7 @@ void construct(value_and_holder &v_h, Alias<Class> *alias_ptr, bool) {
 // holder.  This also handles types like std::shared_ptr<T> and std::unique_ptr<T> where T is a
 // derived type (through those holder's implicit conversion from derived class holder constructors).
 template <typename Class,
-          detail::enable_if_t<!detail::is_smart_holder_type_caster<Cpp<Class>>::value, int> = 0>
+          typename std::enable_if<!detail::is_smart_holder_type_caster<Cpp<Class>>::value, int>::type = 0>
 void construct(value_and_holder &v_h, Holder<Class> holder, bool need_alias) {
     auto *ptr = holder_helper<Holder<Class>>::get(holder);
     no_nullptr(ptr);
@@ -172,7 +172,7 @@ void construct(value_and_holder &v_h, Alias<Class> &&result, bool) {
 //DETAIL/SMART_HOLDER_INIT_H/BEGIN/////////////////////////////////////////////////////////////////
 
 template <typename Class, typename D = std::default_delete<Cpp<Class>>,
-          detail::enable_if_t<detail::is_smart_holder_type_caster<Cpp<Class>>::value, int> = 0>
+          typename std::enable_if<detail::is_smart_holder_type_caster<Cpp<Class>>::value, int>::type = 0>
 void construct(value_and_holder &v_h, std::unique_ptr<Cpp<Class>, D> &&unq_ptr, bool need_alias) {
     auto *ptr = unq_ptr.get();
     no_nullptr(ptr);
@@ -188,7 +188,7 @@ void construct(value_and_holder &v_h, std::unique_ptr<Cpp<Class>, D> &&unq_ptr,
 }
 
 template <typename Class,
-          detail::enable_if_t<detail::is_smart_holder_type_caster<Cpp<Class>>::value, int> = 0>
+          typename std::enable_if<detail::is_smart_holder_type_caster<Cpp<Class>>::value, int>::type = 0>
 void construct(value_and_holder &v_h, std::shared_ptr<Cpp<Class>> &&shd_ptr, bool need_alias) {
     auto *ptr = shd_ptr.get();
     no_nullptr(ptr);

include/pybind11/pybind11.h

@@ -1544,14 +1544,14 @@ class class_ : public detail::generic_type {
 private:
     template <
         typename T = type,
-        detail::enable_if_t<!detail::is_smart_holder_type_caster<T>::value, int> = 0>
+        typename std::enable_if<!detail::is_smart_holder_type_caster<T>::value, int>::type = 0>
     void generic_type_initialize(const detail::type_record &record) {
         generic_type::initialize(record, &detail::type_caster_generic::local_load);
     }
 
     template <
         typename T = type,
-        detail::enable_if_t<detail::is_smart_holder_type_caster<T>::value, int> = 0>
+        typename std::enable_if<detail::is_smart_holder_type_caster<T>::value, int>::type = 0>
     void generic_type_initialize(const detail::type_record &record) {
         generic_type::initialize(record, detail::type_caster<T>::get_local_load_function_ptr());
     }
@@ -1602,7 +1602,7 @@ class class_ : public detail::generic_type {
     /// `.owned`, a new holder will be constructed to manage the value pointer.
     template <
         typename T = type,
-        detail::enable_if_t<!detail::is_smart_holder_type_caster<T>::value, int> = 0>
+        typename std::enable_if<!detail::is_smart_holder_type_caster<T>::value, int>::type = 0>
     static void init_instance(detail::instance *inst, const void *holder_ptr) {
         auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(type)));
         if (!v_h.instance_registered()) {
@@ -1614,7 +1614,7 @@ class class_ : public detail::generic_type {
 
     template <
         typename T = type,
-        detail::enable_if_t<detail::is_smart_holder_type_caster<T>::value, int> = 0>
+        typename std::enable_if<detail::is_smart_holder_type_caster<T>::value, int>::type = 0>
     static void init_instance(detail::instance *inst, const void *holder_ptr) {
         detail::type_caster<T>::template init_instance_for_type<type>(inst, holder_ptr);
     }