TraitListObject gives bad index for __setitem__

[mdickinson]

Given the following code:

>>> from traits.api import *
>>> class A(HasTraits):
...     foo = List(Int)
...     def _foo_items_changed(self, event):
...         print(f"Index: {event.index}; Added: {event.added}; Removed: {event.removed}")
... 
>>> a = A()
>>> a.foo = list(range(10))
>>> a.foo
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

An assignment to a.foo[5:2] inserts new items at position 5. But the emitted TraitListEvent has an index of 2.

>>> a.foo[5:2] = [99]
Index: 2; Added: [99]; Removed: []
>>> a.foo
[0, 1, 2, 3, 4, 99, 5, 6, 7, 8, 9]

[mdickinson]

It's conceivable that this is deliberate bug-for-bug compatibility with existing code. But I don't think so: there's no existing test that fails when I fix this, and it's hard to imagine how the current behaviour could be useful to anyone.

[mdickinson]

Not deliberate: this is a regression from Traits 6.0.0. It needs a fix and a test.