Fix the __imatmul__ method in the array API namespace

Author: asmeurer

numpy/_array_api/_array_object.py

@@ -648,12 +648,21 @@ def __imatmul__(self: array, other: array, /) -> array:
         """
         Performs the operation __imatmul__.
         """
+        # Note: NumPy does not implement __imatmul__.
+
         if isinstance(other, (int, float, bool)):
             # matmul is not defined for scalars, but without this, we may get
             # the wrong error message from asarray.
             other = self._promote_scalar(other)
-        res = self._array.__imatmul__(other._array)
-        return self.__class__._new(res)
+        # __imatmul__ can only be allowed when it would not change the shape
+        # of self.
+        other_shape = other.shape
+        if self.shape == () or other_shape == ():
+            raise ValueError("@= requires at least one dimension")
+        if len(other_shape) == 1 or other_shape[-1] != other_shape[-2]:
+            raise ValueError("@= cannot change the shape of the input array")
+        self._array[:] = self._array.__matmul__(other._array)
+        return self
 
     def __rmatmul__(self: array, other: array, /) -> array:
         """