Make array_t’s converting constructor consistent with other pytypes

* `array_t(const object &)` throws on error
* `array_t::ensure()` is intended for casters
*  `py::isinstance<array_T<T>>()` is intentionally disabled

Author: dean0x7d

include/pybind11/eigen.h

@@ -54,7 +54,7 @@ struct type_caster<Type, enable_if_t<is_eigen_dense<Type>::value && !is_eigen_re
     static constexpr bool isVector = Type::IsVectorAtCompileTime;
 
     bool load(handle src, bool) {
-        array_t<Scalar> buf(src, true);
+        auto buf = array_t<Scalar>::ensure(src);
         if (!buf)
             return false;
 

include/pybind11/numpy.h

@@ -525,11 +525,19 @@ class array : public buffer {
 
 template <typename T, int ExtraFlags = array::forcecast> class array_t : public array {
 public:
-    array_t() : array() { }
+    array_t() = default;
+    array_t(handle h, borrowed_t) : array(h, borrowed) { }
+    array_t(handle h, stolen_t) : array(h, stolen) { }
 
-    array_t(handle h, bool is_borrowed) : array(h, is_borrowed) { m_ptr = ensure_(m_ptr); }
+    PYBIND11_DEPRECATED("Use array_t<T>::ensure() instead")
+    array_t(handle h, bool is_borrowed) : array(raw_array_from_any(h.ptr()), stolen) {
+        if (!m_ptr) PyErr_Clear();
+        if (!is_borrowed) Py_XDECREF(h.ptr());
+    }
 
-    array_t(const object &o) : array(o) { m_ptr = ensure_(m_ptr); }
+    array_t(const object &o) : array(raw_array_from_any(o.ptr()), stolen) {
+        if (!m_ptr) throw error_already_set();
+    }
 
     explicit array_t(const buffer_info& info) : array(info) { }
 
@@ -577,15 +585,24 @@ template <typename T, int ExtraFlags = array::forcecast> class array_t : public
         return *(static_cast<T*>(array::mutable_data()) + get_byte_offset(index...) / itemsize());
     }
 
-    static PyObject *ensure_(PyObject *ptr) {
+    /// Ensure that the argument is a NumPy array of the correct dtype.
+    /// In case of an error, nullptr is returned and the Python error is cleared.
+    static array_t ensure(handle h) {
+        auto result = reinterpret_steal<array_t>(raw_array_from_any(h.ptr()));
+        if (!result)
+            PyErr_Clear();
+        return result;
+    }
+
+    static bool _check(handle) = delete; // Make sure py::instance<array_t<T>>() can't compile.
+
+private:
+    static PyObject *raw_array_from_any(PyObject *ptr) {
         if (ptr == nullptr)
             return nullptr;
         auto& api = detail::npy_api::get();
         PyObject *result = api.PyArray_FromAny_(ptr, pybind11::dtype::of<T>().release().ptr(), 0, 0,
                                                 detail::npy_api::NPY_ENSURE_ARRAY_ | ExtraFlags, nullptr);
-        if (!result)
-            PyErr_Clear();
-        Py_DECREF(ptr);
         return result;
     }
 };
@@ -618,7 +635,7 @@ struct pyobject_caster<array_t<T, ExtraFlags>> {
     using type = array_t<T, ExtraFlags>;
 
     bool load(handle src, bool /* convert */) {
-        value = type(src, true);
+        value = type::ensure(src);
         return static_cast<bool>(value);
     }