API: Add numpy2.h instead and make numpy.h safe

This means that users of `numpy.h` cannot be broken, but need to
update to `numpy2.h` if they want to compile for NumPy 2.

Using Macros simply and didn't bother to try to remove unnecessary
code paths.

CMakeLists.txt

@@ -161,6 +161,7 @@ set(PYBIND11_HEADERS
     include/pybind11/iostream.h
     include/pybind11/functional.h
     include/pybind11/numpy.h
+    include/pybind11/numpy2.h
     include/pybind11/operators.h
     include/pybind11/pybind11.h
     include/pybind11/pytypes.h

include/pybind11/eigen/matrix.h

@@ -9,7 +9,7 @@
 
 #pragma once
 
-#include "../numpy.h"
+#include "../numpy2.h"
 #include "common.h"
 
 /* HINT: To suppress warnings originating from the Eigen headers, use -isystem.

include/pybind11/eigen/tensor.h

@@ -7,7 +7,7 @@
 
 #pragma once
 
-#include "../numpy.h"
+#include "../numpy2.h"
 #include "common.h"
 
 #if defined(__GNUC__) && !defined(__clang__) && !defined(__INTEL_COMPILER)

include/pybind11/numpy.h

@@ -53,17 +53,6 @@ struct handle_type_name<array> {
 template <typename type, typename SFINAE = void>
 struct npy_format_descriptor;
 
-struct PyArrayDescr_Proxy {
-    PyObject_HEAD
-    PyObject *typeobj;
-    char kind;
-    char type;
-    char byteorder;
-    char _former_flags;
-    int type_num;
-    /* Additional fields are NumPy version specific. */
-};
-
 /* NumPy 1 proxy (always includes legacy fields) */
 struct PyArrayDescr1_Proxy {
     PyObject_HEAD
@@ -80,6 +69,22 @@ struct PyArrayDescr1_Proxy {
     PyObject *names;
 };
 
+#ifdef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+struct PyArrayDescr_Proxy {
+    PyObject_HEAD
+    PyObject *typeobj;
+    char kind;
+    char type;
+    char byteorder;
+    char _former_flags;
+    int type_num;
+    /* Additional fields are NumPy version specific. */
+};
+#else
+/* NumPy 1.x only, we can expose all fields */
+typedef PyArrayDescr1_Proxy PyArrayDescr_Proxy;
+#endif
+
 /* NumPy 2 proxy, including legacy fields */
 struct PyArrayDescr2_Proxy {
     PyObject_HEAD
@@ -164,6 +169,13 @@ PYBIND11_NOINLINE module_ import_numpy_core_submodule(const char *submodule_name
     object numpy_version = numpy_lib.attr("NumpyVersion")(version_string);
     int major_version = numpy_version.attr("major").cast<int>();
 
+#ifndef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+    if (major_version >= 2) {
+        throw std::runtime_error("module compiled without NumPy 2 support.  Please modify the "
+                                 "`pybind11/numpy.h` include to `pybind11/numpy2.h` and recompile "
+                                 "(this remains NumPy 1.x compatible but has minor changes).");
+    }
+#endif
     /* `numpy.core` was renamed to `numpy._core` in NumPy 2.0 as it officially
         became a private module. */
     std::string numpy_core_path = major_version >= 2 ? "numpy._core" : "numpy.core";
@@ -276,6 +288,16 @@ struct npy_api {
     PyObject *(*PyArray_FromAny_)(PyObject *, PyObject *, int, int, int, PyObject *);
     int (*PyArray_DescrConverter_)(PyObject *, PyObject **);
     bool (*PyArray_EquivTypes_)(PyObject *, PyObject *);
+#ifndef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+    int (*PyArray_GetArrayParamsFromObject_)(PyObject *,
+                                             PyObject *,
+                                             unsigned char,
+                                             PyObject **,
+                                             int *,
+                                             Py_intptr_t *,
+                                             PyObject **,
+                                             PyObject *);
+#endif
     PyObject *(*PyArray_Squeeze_)(PyObject *);
     // Unused. Not removed because that affects ABI of the class.
     int (*PyArray_SetBaseObject_)(PyObject *, PyObject *);
@@ -302,6 +324,9 @@ struct npy_api {
         API_PyArray_View = 137,
         API_PyArray_DescrConverter = 174,
         API_PyArray_EquivTypes = 182,
+#ifndef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+        API_PyArray_GetArrayParamsFromObject = 278,
+#endif
         API_PyArray_SetBaseObject = 282
     };
 
@@ -644,12 +669,16 @@ class dtype : public object {
     }
 
     /// Size of the data type in bytes.
+#ifndef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+    int itemsize() const { return detail::array_descriptor_proxy(m_ptr)->elsize; }
+#else
     ssize_t itemsize() const {
         if (detail::npy_api::get().PyArray_RUNTIME_VERSION_ < 0x12) {
             return detail::array_descriptor1_proxy(m_ptr)->elsize;
         }
         return detail::array_descriptor2_proxy(m_ptr)->elsize;
     }
+#endif
 
     /// Returns true for structured data types.
     bool has_fields() const {
@@ -686,21 +715,29 @@ class dtype : public object {
     /// Single character for byteorder
     char byteorder() const { return detail::array_descriptor_proxy(m_ptr)->byteorder; }
 
-    /// Alignment of the data type
+/// Alignment of the data type
+#ifndef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+    int alignment() const { return detail::array_descriptor_proxy(m_ptr)->alignment; }
+#else
     ssize_t alignment() const {
         if (detail::npy_api::get().PyArray_RUNTIME_VERSION_ < 0x12) {
-            return detail::array_descriptor1_proxy(m_ptr)->alignment;
+            return detail::array_descriptor2_proxy(m_ptr)->alignment;
         }
-        return detail::array_descriptor2_proxy(m_ptr)->alignment;
+        return detail::array_descriptor1_proxy(m_ptr)->alignment;
     }
+#endif
 
-    /// Flags for the array descriptor
+/// Flags for the array descriptor
+#ifndef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+    char flags() const { return detail::array_descriptor_proxy(m_ptr)->flags; }
+#else
     std::uint64_t flags() const {
         if (detail::npy_api::get().PyArray_RUNTIME_VERSION_ < 0x12) {
             return (unsigned char) detail::array_descriptor1_proxy(m_ptr)->flags;
         }
         return detail::array_descriptor2_proxy(m_ptr)->flags;
     }
+#endif
 
 private:
     static object &_dtype_from_pep3118() {

include/pybind11/numpy2.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#define PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT
+#include "numpy.h"
+#undef PYBIND11_COMPILE_WITH_NUMPY2_SUPPORT

tests/extra_python_package/test_files.py

@@ -38,6 +38,7 @@
     "include/pybind11/gil_safe_call_once.h",
     "include/pybind11/iostream.h",
     "include/pybind11/numpy.h",
+    "include/pybind11/numpy2.h",
     "include/pybind11/operators.h",
     "include/pybind11/options.h",
     "include/pybind11/pybind11.h",