BUG: fix __setitem__ for invalid dtype combinations

Do not allow e.g.

>>> a = xp.asarray([1, 2, 3])
>>> a[0] = 3.5     # cannot type-promote a float scalar and an int array
>>> a[0] = xp.asarray(3.5)   # cannot integer and float arrays cannot be promoted together

Author: ev-br

array_api_strict/_array_object.py

@@ -942,6 +942,15 @@ def __setitem__(
         self._validate_index(key, op="setitem")
         # Indexing self._array with array_api_strict arrays can be erroneous
         np_key = key._array if isinstance(key, Array) else key
+
+        # sanitize the value
+        other = value
+        if isinstance(value, (bool, int, float, complex)):
+            other = self._promote_scalar(value)
+        dt = _result_type(self.dtype, other.dtype)
+        if dt != self.dtype:
+            raise TypeError(f"mismatched dtypes: {self.dtype = } and {other.dtype = }")
+
         self._array.__setitem__(np_key, asarray(value)._array)
 
     def __sub__(self, other: Array | complex, /) -> Array:

array_api_strict/tests/test_array_object.py

@@ -18,12 +18,14 @@
     _integer_or_boolean_dtypes,
     _real_numeric_dtypes,
     _numeric_dtypes,
+    uint8,
     int8,
     int16,
     int32,
     int64,
     uint64,
     float64,
+    complex128,
     bool as bool_,
 )
 from .._flags import set_array_api_strict_flags
@@ -193,6 +195,29 @@ def test_indexing_arrays_different_devices():
         a[idx1, idx2]
 
 
+def test_setitem_invalid_promotions():
+    # Check that violating these two raises:
+    #   Setting array values must not affect the data type of self, and
+    #   Behavior must otherwise follow Type Promotion Rules.
+    a = asarray([1, 2, 3])
+    with pytest.raises(TypeError):
+        a[0] = 3.5
+
+    with pytest.raises(TypeError):
+        a[0] = asarray(3.5)
+
+    a = asarray([1, 2, 3], dtype=uint8)
+    with pytest.raises(TypeError):
+        a[0] = asarray(42, dtype=uint64)
+
+    a = asarray([1, 2, 3], dtype=float64)
+    with pytest.raises(TypeError):
+        a[0] = 3.5j
+
+    with pytest.raises(TypeError):
+        a[0] = asarray(3.5j, dtype=complex128)
+
+
 def test_promoted_scalar_inherits_device():
     device1 = Device("device1")
     x = asarray([1., 2, 3], device=device1)