From a3a7b39f5cf466ef19c05070ef839105836c6674 Mon Sep 17 00:00:00 2001
From: Eric Cousineau <eric.cousineau@tri.global>
Date: Sun, 10 Apr 2022 20:27:45 -0400
Subject: [PATCH 1/2] cast: Add is_generic_type<T>

---
 include/pybind11/cast.h    | 9 +++++++++
 include/pybind11/pytypes.h | 9 ++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index 07a56e71f..a1efb5ed5 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -36,6 +36,15 @@ class type_caster : public type_caster_base<type> {};
 template <typename type>
 using make_caster = type_caster<intrinsic_t<type>>;
 
+template <typename T>
+struct is_generic_type<T, enable_if_t<std::is_base_of<type_caster_generic, make_caster<T>>::value>>
+    : public std::true_type {};
+
+template <typename T>
+struct is_generic_type<T,
+                       enable_if_t<!std::is_base_of<type_caster_generic, make_caster<T>>::value>>
+    : public std::false_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) {
diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index ba0fda0a8..cf92e6827 100644
--- a/include/pybind11/pytypes.h
+++ b/include/pybind11/pytypes.h
@@ -38,6 +38,11 @@ PYBIND11_NAMESPACE_BEGIN(detail)
 class args_proxy;
 bool isinstance_generic(handle obj, const std::type_info &tp);
 
+// Indicates that type is generic and and does not have a specialized
+// `type_caster<>` specialization. Defined in `cast.h`.
+template <typename T, typename SFINAE = void>
+struct is_generic_type;
+
 // Accessor forward declarations
 template <typename Policy>
 class accessor;
@@ -476,7 +481,7 @@ inline void raise_from(error_already_set &err, PyObject *type, const char *messa
 /** \ingroup python_builtins
     \rst
     Return true if ``obj`` is an instance of ``T``. Type ``T`` must be a subclass of
-    `object` or a class which was exposed to Python as ``py::class_<T>``.
+    `object` or a class which was exposed to Python as ``py::class_<T>`` (generic).
 \endrst */
 template <typename T, detail::enable_if_t<std::is_base_of<object, T>::value, int> = 0>
 bool isinstance(handle obj) {
@@ -485,6 +490,8 @@ bool isinstance(handle obj) {
 
 template <typename T, detail::enable_if_t<!std::is_base_of<object, T>::value, int> = 0>
 bool isinstance(handle obj) {
+    static_assert(detail::is_generic_type<T>::value,
+                  "isisntance<T>() requires specialization for this type");
     return detail::isinstance_generic(obj, typeid(T));
 }
 

From 58d4aaeedc7adeb3f9861c1ac6f574bd12421933 Mon Sep 17 00:00:00 2001
From: Eric Cousineau <eric.cousineau@tri.global>
Date: Mon, 11 Apr 2022 14:15:11 -0400
Subject: [PATCH 2/2] Update include/pybind11/pytypes.h

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
---
 include/pybind11/pytypes.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index cf92e6827..7658142fa 100644
--- a/include/pybind11/pytypes.h
+++ b/include/pybind11/pytypes.h
@@ -491,7 +491,7 @@ bool isinstance(handle obj) {
 template <typename T, detail::enable_if_t<!std::is_base_of<object, T>::value, int> = 0>
 bool isinstance(handle obj) {
     static_assert(detail::is_generic_type<T>::value,
-                  "isisntance<T>() requires specialization for this type");
+                  "isinstance<T>() requires specialization for this type");
     return detail::isinstance_generic(obj, typeid(T));
 }