>>> class Dict(dict):
       
	def keys(self): assert 0
	def update(*args, **kwds): assert 0
	def __getitem__(self, key): assert 0
        def __iter__(self): assert 0 

	
>>> {**Dict(a=1)}
{'a': 1}

The opcode uses PyDict_Update, which calls the internal dict_merge function which contains the following line:

    if (PyDict_Check(b) && (Py_TYPE(b)->tp_iter == (getiterfunc)dict_iter))

Translated to Python, that should be equal to 

    if type(b).__flags__ & (1<<29) and type.__getattribute__(type(b), '__iter__') is type.__getattribute__(dict, '__iter__')`

Both that and the line in C evaluate to false for me (while a dict subclass that doesn't override __iter__ evaluates to true), so I 
 apparently can't help narrow down the problem any further assuming people agree that this is even is a problem...

The BUILD_MAP_UNPACK_WITH_CALL, CALL_FUNCTION_EX, and CALL_FUNCTION_KW opcodes are affected as well.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in keys
AssertionError

This works as expected to me.

You say it doesn't work as expected, but you don't say what you expect or why. (Don't make me guess what you mean -- explicit is better than implicit.)

When I try your subclass in 3.6, I get an unexpected TypeError:

py> class Dict(dict):
...     def keys(self): assert 0
...     def update(*args, **kwds): assert 0
...     def __getitem__(self, key): assert 0
...     def __iter__(self): assert 0
...
py> {**Dict(a=1)}
{'a': 1}
py> Dict(a=1).keys()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in keys
TypeError

How weird... after restarting the interpreter, I can't reproduce that TypeError. I get the AssertionError Serhiy showed.

BUILD_MAP_UNPACK and BUILD_MAP_UNPACK_WITH_CALL have been removed.