BUG: Promotion between strings and objects was assymetric

After an unrelated fix, the logic for string and object promotion
was incorrect briefly, this fixes it to be correct (symmetric).
Before, string and unicode would return that
`string.__common_dtype(object)` is in fact `string`, which is of
course incorrect. (since `object.__common_dtype__(other)` always
returns `object` this depended on the order, and the NumPy tests
apparently did only test the opposite direction (or nothing).

2 files changed, 38 insertions, 35 deletions

diff --git a/numpy/core/src/multiarray/dtypemeta.c b/numpy/core/src/multiarray/dtypemeta.c
index b1cd074a0..2931977c2 100644
--- a/numpy/core/src/multiarray/dtypemeta.c
+++ b/numpy/core/src/multiarray/dtypemeta.c
@@ -400,9 +400,10 @@ default_builtin_common_dtype(PyArray_DTypeMeta *cls, PyArray_DTypeMeta *other)
 static PyArray_DTypeMeta *
 string_unicode_common_dtype(PyArray_DTypeMeta *cls, PyArray_DTypeMeta *other)
 {
-    assert(cls->type_num < NPY_NTYPES);
+    assert(cls->type_num < NPY_NTYPES && cls != other);
     if (!other->legacy || (!PyTypeNum_ISNUMBER(other->type_num) &&
-            (cls->type_num == NPY_STRING && other->type_num == NPY_UNICODE))) {
+            /* Not numeric so defer unless cls is unicode and other is string */
+            !(cls->type_num == NPY_UNICODE && other->type_num == NPY_STRING))) {
         Py_INCREF(Py_NotImplemented);
         return (PyArray_DTypeMeta *)Py_NotImplemented;
     }
diff --git a/numpy/core/tests/test_numeric.py b/numpy/core/tests/test_numeric.py
index 866a96e31..280874d21 100644
--- a/numpy/core/tests/test_numeric.py
+++ b/numpy/core/tests/test_numeric.py
@@ -888,39 +888,41 @@ class TestTypes:
         assert np.can_cast(rational_dt, double_dt)
         assert np.promote_types(double_dt, rational_dt) is double_dt
 
-    def test_promote_types_strings(self):
-        assert_equal(np.promote_types('bool', 'S'), np.dtype('S5'))
-        assert_equal(np.promote_types('b', 'S'), np.dtype('S4'))
-        assert_equal(np.promote_types('u1', 'S'), np.dtype('S3'))
-        assert_equal(np.promote_types('u2', 'S'), np.dtype('S5'))
-        assert_equal(np.promote_types('u4', 'S'), np.dtype('S10'))
-        assert_equal(np.promote_types('u8', 'S'), np.dtype('S20'))
-        assert_equal(np.promote_types('i1', 'S'), np.dtype('S4'))
-        assert_equal(np.promote_types('i2', 'S'), np.dtype('S6'))
-        assert_equal(np.promote_types('i4', 'S'), np.dtype('S11'))
-        assert_equal(np.promote_types('i8', 'S'), np.dtype('S21'))
-        assert_equal(np.promote_types('bool', 'U'), np.dtype('U5'))
-        assert_equal(np.promote_types('b', 'U'), np.dtype('U4'))
-        assert_equal(np.promote_types('u1', 'U'), np.dtype('U3'))
-        assert_equal(np.promote_types('u2', 'U'), np.dtype('U5'))
-        assert_equal(np.promote_types('u4', 'U'), np.dtype('U10'))
-        assert_equal(np.promote_types('u8', 'U'), np.dtype('U20'))
-        assert_equal(np.promote_types('i1', 'U'), np.dtype('U4'))
-        assert_equal(np.promote_types('i2', 'U'), np.dtype('U6'))
-        assert_equal(np.promote_types('i4', 'U'), np.dtype('U11'))
-        assert_equal(np.promote_types('i8', 'U'), np.dtype('U21'))
-        assert_equal(np.promote_types('bool', 'S1'), np.dtype('S5'))
-        assert_equal(np.promote_types('bool', 'S30'), np.dtype('S30'))
-        assert_equal(np.promote_types('b', 'S1'), np.dtype('S4'))
-        assert_equal(np.promote_types('b', 'S30'), np.dtype('S30'))
-        assert_equal(np.promote_types('u1', 'S1'), np.dtype('S3'))
-        assert_equal(np.promote_types('u1', 'S30'), np.dtype('S30'))
-        assert_equal(np.promote_types('u2', 'S1'), np.dtype('S5'))
-        assert_equal(np.promote_types('u2', 'S30'), np.dtype('S30'))
-        assert_equal(np.promote_types('u4', 'S1'), np.dtype('S10'))
-        assert_equal(np.promote_types('u4', 'S30'), np.dtype('S30'))
-        assert_equal(np.promote_types('u8', 'S1'), np.dtype('S20'))
-        assert_equal(np.promote_types('u8', 'S30'), np.dtype('S30'))
+    @pytest.mark.parametrize("swap", ["", "swap"])
+    @pytest.mark.parametrize("string_dtype", ["U", "S"])
+    def test_promote_types_strings(self, swap, string_dtype):
+        if swap == "swap":
+            promote_types = lambda a, b: np.promote_types(b, a)
+        else:
+            promote_types = np.promote_types
+
+        S = string_dtype
+        # Promote numeric with unsized string:
+        assert_equal(promote_types('bool', S), np.dtype(S+'5'))
+        assert_equal(promote_types('b', S), np.dtype(S+'4'))
+        assert_equal(promote_types('u1', S), np.dtype(S+'3'))
+        assert_equal(promote_types('u2', S), np.dtype(S+'5'))
+        assert_equal(promote_types('u4', S), np.dtype(S+'10'))
+        assert_equal(promote_types('u8', S), np.dtype(S+'20'))
+        assert_equal(promote_types('i1', S), np.dtype(S+'4'))
+        assert_equal(promote_types('i2', S), np.dtype(S+'6'))
+        assert_equal(promote_types('i4', S), np.dtype(S+'11'))
+        assert_equal(promote_types('i8', S), np.dtype(S+'21'))
+        # Promote numeric with sized string:
+        assert_equal(promote_types('bool', S+'1'), np.dtype(S+'5'))
+        assert_equal(promote_types('bool', S+'30'), np.dtype(S+'30'))
+        assert_equal(promote_types('b', S+'1'), np.dtype(S+'4'))
+        assert_equal(promote_types('b', S+'30'), np.dtype(S+'30'))
+        assert_equal(promote_types('u1', S+'1'), np.dtype(S+'3'))
+        assert_equal(promote_types('u1', S+'30'), np.dtype(S+'30'))
+        assert_equal(promote_types('u2', S+'1'), np.dtype(S+'5'))
+        assert_equal(promote_types('u2', S+'30'), np.dtype(S+'30'))
+        assert_equal(promote_types('u4', S+'1'), np.dtype(S+'10'))
+        assert_equal(promote_types('u4', S+'30'), np.dtype(S+'30'))
+        assert_equal(promote_types('u8', S+'1'), np.dtype(S+'20'))
+        assert_equal(promote_types('u8', S+'30'), np.dtype(S+'30'))
+        # Promote with object:
+        assert_equal(promote_types('O', S+'30'), np.dtype('O'))
 
     @pytest.mark.parametrize(["dtype1", "dtype2"],
             [[np.dtype("V6"), np.dtype("V10")],