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If I try to pickle and unpickle an object of a class that has specialized a generic superclass, when I try to unpickle I get this error:
TypeError: descriptor '__dict__' for 'A' objects doesn't apply to 'B' object
Test case:
from typing import Generic, TypeVar
import pickle
T = TypeVar("T")
class A(Generic[T]):
 def __init__(self, x: T):
 self.x = x
class B(A[str]):
 def __init__(self, x: str):
 self.x = x
b = B("hello")
z = pickle.dumps(b)
print(z)
_ = pickle.loads(z)

FWIW the pickle produced looks like this, decoded with pickletools.dis():
 0: \x80 PROTO 3
 2: c GLOBAL '__main__ B'
 14: q BINPUT 0
 16: ) EMPTY_TUPLE
 17: \x81 NEWOBJ
 18: q BINPUT 1
 20: } EMPTY_DICT
 21: q BINPUT 2
 23: X BINUNICODE 'x'
 29: q BINPUT 3
 31: X BINUNICODE 'hello'
 41: q BINPUT 4
 43: s SETITEM
 44: b BUILD
 45: . STOP

The issue seems to be the line
 inst_dict = inst.__dict__
in _Unpickler.load_build(). (I found this out by forcing sys.modules['_pickle'] = None, so the pure-Python pickle.py code gets used.)
This leads to a simpler repro: 
# Use the same class definitions for A and B
b = B("hello")
print(b.__dict__) # Same error message as before
That is, specialized subclasses of generics don't have a working __dict__ attribute! Interestingly, A("hello").__dict__ works.
I have to ponder this more.

Update: I have not forgotten this, but it's a tough case and I haven't made much progress. I don't think I'll make the deadline for 3.5.1.

Actually, I just found an embarrassingly simple fix:
diff --git a/src/typing.py b/src/typing.py
index d900036..49c4a06 100644
--- a/src/typing.py
+++ b/src/typing.py
@@ -981,7 +981,7 @@ class GenericMeta(TypingMeta, abc.ABCMeta):
 "Cannot substitute %s for %s in %s" %
 (_type_repr(new), _type_repr(old), self))
 
- return self.__class__(self.__name__, self.__bases__,
+ return self.__class__(self.__name__, (self,) + self.__bases__,
 dict(self.__dict__),
 parameters=params,
 origin=self,
I'll go check this in now, together with a test.

New changeset 1d37f7dbbb37 by Guido van Rossum in branch '3.5':
Issue #25472: In B[<type>], insert B in front of __bases__, to make the __dict__ descriptor work.
https://hg.python.org/cpython/rev/1d37f7dbbb37
New changeset 63c261185fc7 by Guido van Rossum in branch 'default':
Issue #25472: In B[<type>], insert B in front of __bases__, to make the __dict__ descriptor work. (Merge 3.5->3.6)
https://hg.python.org/cpython/rev/63c261185fc7 

Thanks for the report! Made it before the 3.5.1 (rc1) deadline.