DEP: Expire deprecation of dtype/signature allowing instances

We never really allowed instances here and deprecated it since
NumPy 1.21 (it just failed completely in 1.21.0).

numpy/core/src/umath/ufunc_object.c

@@ -4361,23 +4361,16 @@ _get_dtype(PyObject *dtype_obj) {
         }
         else if (NPY_UNLIKELY(out->singleton != descr)) {
             /* This does not warn about `metadata`, but units is important. */
-            if (!PyArray_EquivTypes(out->singleton, descr)) {
-                /* Deprecated NumPy 1.21.2 (was an accidental error in 1.21) */
-                if (DEPRECATE(
+            if (out->singleton == NULL
+                    || !PyArray_EquivTypes(out->singleton, descr)) {
+                PyErr_SetString(PyExc_TypeError,
                         "The `dtype` and `signature` arguments to "
                         "ufuncs only select the general DType and not details "
-                        "such as the byte order or time unit (with rare "
-                        "exceptions see release notes).  To avoid this warning "
-                        "please use the scalar types `np.float64`, or string "
-                        "notation.\n"
-                        "In rare cases where the time unit was preserved, "
-                        "either cast the inputs or provide an output array. "
-                        "In the future NumPy may transition to allow providing "
-                        "`dtype=` to denote the outputs `dtype` as well. "
-                        "(Deprecated NumPy 1.21)") < 0) {
-                    Py_DECREF(descr);
-                    return NULL;
-                }
+                        "such as the byte order or time unit. "
+                        "You can avoid this error by using the scalar types "
+                        "`np.float64` or the dtype string notation.");
+                Py_DECREF(descr);
+                return NULL;
             }
         }
         Py_INCREF(out);

numpy/core/tests/test_ufunc.py

@@ -3,6 +3,7 @@
 import sys
 
 import pytest
+from pytest import param
 
 import numpy as np
 import numpy.core._umath_tests as umt
@@ -477,6 +478,34 @@ def test_signature_dtype_type(self):
         float_dtype = type(np.dtype(np.float64))
         np.add(3, 4, signature=(float_dtype, float_dtype, None))
 
+    @pytest.mark.parametrize("get_kwarg", [
+            lambda dt: dict(dtype=x),
+            lambda dt: dict(signature=(x, None, None))])
+    def test_signature_dtype_instances_allowed(self, get_kwarg):
+        # We allow certain dtype instances when there is a clear singleton
+        # and the given one is equivalent; mainly for backcompat.
+        int64 = np.dtype("int64")
+        int64_2 = pickle.loads(pickle.dumps(int64))
+        # Relies on pickling behavior, if assert fails just remove test...
+        assert int64 is not int64_2
+
+        assert np.add(1, 2, **get_kwarg(int64_2)).dtype == int64
+        td = np.timedelta(2, "s")
+        assert np.add(td, td, **get_kwarg("m8")).dtype == "m8[s]"
+
+    @pytest.mark.parametrize("get_kwarg", [
+            param(lambda x: dict(dtype=x), id="dtype"),
+            param(lambda x: dict(signature=(x, None, None)), id="signature")])
+    def test_signature_dtype_instances_allowed(self, get_kwarg):
+        msg = "The `dtype` and `signature` arguments to ufuncs"
+
+        with pytest.raises(TypeError, match=msg):
+            np.add(3, 5, **get_kwarg(np.dtype("int64").newbyteorder()))
+        with pytest.raises(TypeError, match=msg):
+            np.add(3, 5, **get_kwarg(np.dtype("m8[ns]")))
+        with pytest.raises(TypeError, match=msg):
+            np.add(3, 5, **get_kwarg("m8[ns]"))
+
     @pytest.mark.parametrize("casting", ["unsafe", "same_kind", "safe"])
     def test_partial_signature_mismatch(self, casting):
         # If the second argument matches already, no need to specify it: