API: Disallow strings in logical ufuncs

numpy/core/src/umath/dispatching.c

@@ -1018,6 +1018,11 @@ logical_ufunc_promoter(PyUFuncObject *NPY_UNUSED(ufunc),
         /* bail out, this is _only_ to give future/deprecation warning! */
         return -1;
     }
+    if ((op_dtypes[0] != NULL && PyTypeNum_ISSTRING(op_dtypes[0]->type_num))
+            || PyTypeNum_ISSTRING(op_dtypes[1]->type_num)) {
+        /* bail out on strings: currently casting them to bool is too weird */
+        return -1;
+    }
 
     for (int i = 0; i < 3; i++) {
         PyArray_DTypeMeta *item;

numpy/core/tests/test_ufunc.py

@@ -2134,7 +2134,7 @@ def test_logical_ufuncs_mixed_object_signatures(self, ufunc, signature):
             [np.logical_and, np.logical_or, np.logical_xor])
     def test_logical_ufuncs_support_anything(self, ufunc):
         # The logical ufuncs support even input that can't be promoted:
-        a = np.array('1')
+        a = np.array(b'1', dtype="V3")
         c = np.array([1., 2.])
         assert_array_equal(ufunc(a, c), ufunc([True, True], True))
         assert ufunc.reduce(a) == True
@@ -2150,6 +2150,23 @@ def test_logical_ufuncs_support_anything(self, ufunc):
         out = np.zeros((), dtype=a.dtype)
         assert ufunc.reduce(a, out=out) == 1
 
+    @pytest.mark.parametrize("ufunc",
+            [np.logical_and, np.logical_or, np.logical_xor])
+    def test_logical_ufuncs_reject_string(self, ufunc):
+        """
+        Logical ufuncs are normally well defined by working with the boolean
+        equivalent, i.e. casting all inputs to bools should work.
+
+        However, casting strings to bools is *currently* weird, because it
+        actually uses `bool(int(str))`.  Thus we explicitly reject strings.
+        This test should succeed (and can probably just be removed) as soon as
+        string to bool casts are well defined in NumPy.
+        """
+        with pytest.raises(TypeError, match="contain a loop with signature"):
+            ufunc(["1"], ["3"])
+        with pytest.raises(TypeError, match="contain a loop with signature"):
+            ufunc.reduce(["1", "2", "0"])
+
     @pytest.mark.parametrize("ufunc",
              [np.logical_and, np.logical_or, np.logical_xor])
     def test_logical_ufuncs_out_cast_check(self, ufunc):