From b9892fad768418e3c8856165166f66d9e6c0a3f9 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 15 Feb 2022 13:32:30 -0800
Subject: [PATCH 1/7] Removing C++20 condition for MSVC is_template_base_of
 decltype workaround.

---
 include/pybind11/detail/common.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 1070d686ec..4565e30a1c 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -822,7 +822,7 @@ struct is_template_base_of_impl {
 /// Check if a template is the base of a type. For example:
 /// `is_template_base_of<Base, T>` is true if `struct T : Base<U> {}` where U can be anything
 template <template <typename...> class Base, typename T>
-#if defined(PYBIND11_CPP20) || !defined(_MSC_VER) // Sadly, all MSVC versions incl. 2022 need this.
+#if !defined(_MSC_VER) // Sadly, all MSVC versions incl. 2022 need this.
 using is_template_base_of
     = decltype(is_template_base_of_impl<Base>::check((intrinsic_t<T> *) nullptr));
 #else

From a6dd79386584c43d260e210d5da1912c35dffc76 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 15 Feb 2022 13:33:28 -0800
Subject: [PATCH 2/7] `-DDOWNLOAD_EIGEN=ON` for MSVC 2022 C++20

---
 .github/workflows/ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6bf56c4e30..47e32b8653 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -835,7 +835,7 @@ jobs:
         cmake -S . -B build
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=OFF
+        -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=20
 
     - name: Build C++20

From c45b469a560421c29514ef57c2e4d88fccce3fe2 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 15 Feb 2022 13:38:03 -0800
Subject: [PATCH 3/7] `-DDOWNLOAD_EIGEN=ON` for MSVC 2019 C++20

---
 .github/workflows/ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 47e32b8653..5944f6eaf4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -712,7 +712,7 @@ jobs:
 
         include:
           - python: 3.9
-            args: -DCMAKE_CXX_STANDARD=20 -DDOWNLOAD_EIGEN=OFF
+            args: -DCMAKE_CXX_STANDARD=20 -DDOWNLOAD_EIGEN=ON
           - python: 3.8
             args: -DCMAKE_CXX_STANDARD=17
 

From e263cda43de3564f57725ed2072cf3b34c22462b Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 15 Feb 2022 14:13:54 -0800
Subject: [PATCH 4/7] `-DPYBIND11_WERROR=OFF` for MSVC C++20 (2019, 2020)

---
 .github/workflows/ci.yml | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5944f6eaf4..17dbbe144f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -712,7 +712,7 @@ jobs:
 
         include:
           - python: 3.9
-            args: -DCMAKE_CXX_STANDARD=20 -DDOWNLOAD_EIGEN=ON
+            args: -DCMAKE_CXX_STANDARD=20 -DDOWNLOAD_EIGEN=ON -DPYBIND11_WERROR=OFF
           - python: 3.8
             args: -DCMAKE_CXX_STANDARD=17
 
@@ -837,6 +837,7 @@ jobs:
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=20
+        -DPYBIND11_WERROR=OFF
 
     - name: Build C++20
       run: cmake --build build -j 2

From 533e5ad4b8e9a3712ba648559003a7db4dfe99a9 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 15 Feb 2022 14:45:36 -0800
Subject: [PATCH 5/7] Restoring `defined(PYBIND11_CPP20)` in common.h

---
 include/pybind11/detail/common.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 4565e30a1c..1070d686ec 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -822,7 +822,7 @@ struct is_template_base_of_impl {
 /// Check if a template is the base of a type. For example:
 /// `is_template_base_of<Base, T>` is true if `struct T : Base<U> {}` where U can be anything
 template <template <typename...> class Base, typename T>
-#if !defined(_MSC_VER) // Sadly, all MSVC versions incl. 2022 need this.
+#if defined(PYBIND11_CPP20) || !defined(_MSC_VER) // Sadly, all MSVC versions incl. 2022 need this.
 using is_template_base_of
     = decltype(is_template_base_of_impl<Base>::check((intrinsic_t<T> *) nullptr));
 #else

From cbb29c0b51d8976ba6e0b5f867c85bbaaaa56393 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 15 Feb 2022 16:16:03 -0800
Subject: [PATCH 6/7] pragma warning(disable : 5054) in eigen.h

---
 include/pybind11/detail/common.h | 4 +++-
 include/pybind11/eigen.h         | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 1070d686ec..1d4939bed7 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -822,7 +822,9 @@ struct is_template_base_of_impl {
 /// Check if a template is the base of a type. For example:
 /// `is_template_base_of<Base, T>` is true if `struct T : Base<U> {}` where U can be anything
 template <template <typename...> class Base, typename T>
-#if defined(PYBIND11_CPP20) || !defined(_MSC_VER) // Sadly, all MSVC versions incl. 2022 need this.
+// Sadly, all MSVC versions incl. 2022 need the workaround, even in C++20 mode.
+// See also: https://github.com/pybind/pybind11/pull/3741
+#if !defined(_MSC_VER)
 using is_template_base_of
     = decltype(is_template_base_of_impl<Base>::check((intrinsic_t<T> *) nullptr));
 #else
diff --git a/include/pybind11/eigen.h b/include/pybind11/eigen.h
index 47820ce895..930c4f7fa8 100644
--- a/include/pybind11/eigen.h
+++ b/include/pybind11/eigen.h
@@ -21,10 +21,12 @@
 // make it version specific, or even remove it later, but considering that
 // 1. C4127 is generally far more distracting than useful for modern template code, and
 // 2. we definitely want to ignore any MSVC warnings originating from Eigen code,
-// it is probably best to keep this around indefinitely.
+//    it is probably best to keep this around indefinitely.
 #if defined(_MSC_VER)
 #    pragma warning(push)
 #    pragma warning(disable : 4127) // C4127: conditional expression is constant
+#    pragma warning(disable : 5054) // https://github.com/pybind/pybind11/pull/3741
+//       C5054: operator '&': deprecated between enumerations of different types
 #endif
 
 #include <Eigen/Core>

From 7301a868ed479d0d47d8a57541bf9dd2c31a16c5 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Tue, 15 Feb 2022 16:18:32 -0800
Subject: [PATCH 7/7] Reverting `-DPYBIND11_WERROR=OFF` changes.

---
 .github/workflows/ci.yml | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 17dbbe144f..bc31281db5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -712,7 +712,7 @@ jobs:
 
         include:
           - python: 3.9
-            args: -DCMAKE_CXX_STANDARD=20 -DDOWNLOAD_EIGEN=ON -DPYBIND11_WERROR=OFF
+            args: -DCMAKE_CXX_STANDARD=20
           - python: 3.8
             args: -DCMAKE_CXX_STANDARD=17
 
@@ -837,7 +837,6 @@ jobs:
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=20
-        -DPYBIND11_WERROR=OFF
 
     - name: Build C++20
       run: cmake --build build -j 2