PEP 448 defines unpacking generalizations for tuples.  However, this does not currently work for subscripted tuples that are not delimited by parentheses.

Current behavior (Tested on 3.6/3.7a4):
>>> class Subscriptable:
...     def __getitem__(self, item):
...         return item
...
>>> ss = Subscriptable()
>>> 
>>> 1, 2, 3
(1, 2, 3)
>>> ss[1, 2, 3]
(1, 2, 3)
>>> *range(5), 42
(0, 1, 2, 3, 4, 42)
>>> ss[*range(5), 42]  # This should be the same as above
  File "<stdin>", line 1
    ss[*range(5), 42]
       ^
SyntaxError: invalid syntax
>>> ss[(*range(5), 42)]  # Workaround
(0, 1, 2, 3, 4, 42)

Tuples often need parentheses when embedded in expressions.  So often that beginners often think that they are required.  I believe this should be closed as 'not a bug'.  I nosied the two authors of the implementation for their opinion.

Yeah, but in this case, you don't need parentheses unless you use unpacking.  That is unexpected behavior.

This came up already on python-ideas: https://groups.google.com/forum/#!topic/python-ideas/YOpT9fDQyFk

I think this was an oversight, and I'm with Ben that it's unexpected.  That said, this is usually the kind of thing that Guido likes to comment on.  My suggestion is to first check to see if you can change the grammar, and then ask Guido what he thinks.

Issue32117 looks considering a more general question about iterable unpacking.