The exact same issue happens when concatenating a list subclass to a tuple:

>>> () + L([1,2])
[1, 2]
>>> (3,) + L([1,2])
# crash

Also, note that in this case a list is returned, not a tuple despite the first operand being a tuple. Conversely:

>>> [] + T((1,2))
(1, 2)

A tuple is returned, not a list. Which is exactly what happens when calling T.__add__:

>>> T.__add__((), T((1,2)))
(1, 2)

My intuition is that the issue is somewhere in binary_op1() (called by PyNumber_Add) in abstract.c. I can't go much further since my own builds don't exhibit the issue.

(in the end, chances are it's a MSVC compiler bug)