Merge pull request #20617 from charris/backport-20616

MAINT: Help boost::python libraries at least not crash

numpy/core/src/multiarray/multiarraymodule.c

@@ -1478,6 +1478,24 @@ PyArray_EquivTypes(PyArray_Descr *type1, PyArray_Descr *type2)
     if (type1 == type2) {
         return 1;
     }
+
+    if (Py_TYPE(Py_TYPE(type1)) == &PyType_Type) {
+        /*
+         * 2021-12-17: This case is nonsense and should be removed eventually!
+         *
+         * boost::python has/had a bug effectively using EquivTypes with
+         * `type(arbitrary_obj)`.  That is clearly wrong as that cannot be a
+         * `PyArray_Descr *`.  We assume that `type(type(type(arbitrary_obj))`
+         * is always in practice `type` (this is the type of the metaclass),
+         * but for our descriptors, `type(type(descr))` is DTypeMeta.
+         *
+         * In that case, we just return False.  There is a possibility that
+         * this actually _worked_ effectively (returning 1 sometimes).
+         * We ignore that possibility for simplicity; it really is not our bug.
+         */
+        return 0;
+    }
+
     /*
      * Do not use PyArray_CanCastTypeTo because it supports legacy flexible
      * dtypes as input.