Workaround NVCC parse failure in cast_op (google#4893)

* Workaround NVCC parse failure in `cast_op`

There is a bug in some CUDA versions (observed in CUDA 12.1 and 11.7 w/ GCC 12.2),
that makes `cast_op` fail to compile:
  `cast.h:45:120: error: expected template-name before ‘<’ token`

Defining the nested type as an alias and using it allows this to work
without any change in semantics.

Fixes google#4606

* style: pre-commit fixes

* Add comments to result_t referencing PR

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/pybind11/cast.h

@@ -42,13 +42,15 @@ using make_caster = type_caster<intrinsic_t<type>>;
 // Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
 template <typename T>
 typename make_caster<T>::template cast_op_type<T> cast_op(make_caster<T> &caster) {
-    return caster.operator typename make_caster<T>::template cast_op_type<T>();
+    using result_t = typename make_caster<T>::template cast_op_type<T>; // See PR #4893
+    return caster.operator result_t();
 }
 template <typename T>
 typename make_caster<T>::template cast_op_type<typename std::add_rvalue_reference<T>::type>
 cast_op(make_caster<T> &&caster) {
-    return std::move(caster).operator typename make_caster<T>::
-        template cast_op_type<typename std::add_rvalue_reference<T>::type>();
+    using result_t = typename make_caster<T>::template cast_op_type<
+        typename std::add_rvalue_reference<T>::type>; // See PR #4893
+    return std::move(caster).operator result_t();
 }
 
 template <typename type>