while fixing bpo-39215, I noticed that there seems to be a place here where posonlyargs was missed: https://github.com/python/cpython/blob/7dc72b8d4f2c9d1eed20f314fd6425eab66cbc89/Python/ast_opt.c#L617-L627

not sure if this is intentional or not -- happy to make a patch which adds a line there if someone can help me with the test

Hi Anthony,

> not sure if this is intentional or not

Thanks for the catch! Is not intentional. Could you made a PR adding the line?

> Happy to make a patch which adds a line there if someone can help me with the test

We do not have explicit tests for constant folding in the ast level an, in this case, is going to be folded anyway via another code path. For example:

def g():
    def f(x : 3 in {1,2,3}, /): ...
    return f

print(g.__code__.co_consts[2])
frozenset({1, 2, 3})

If you want to add a test, you could check the dis output for this example:

def make_func(x):
     def f(x : not  (x is x), /): ...

In this case the compiler emits:

  3           0 LOAD_GLOBAL              0 (x)
              2 LOAD_GLOBAL              0 (x)
              4 COMPARE_OP               8 (is)
              6 UNARY_NOT
              8 LOAD_CONST               1 (('x',))
             10 BUILD_CONST_KEY_MAP      1
             12 LOAD_CONST               2 (<code object f at 0x7f3754f71040, file "/home/pablogsal/github/python/master/lel.py", line 3>)
             14 LOAD_CONST               3 ('code.<locals>.f')
             16 MAKE_FUNCTION            4 (annotations)
             18 STORE_FAST               0 (f)

while for 

def make_func(x):
     def f(x : not  (x is x)): .

emits more efficient code (using 'not is' instead of is and UNARY_NOT:

  4          20 LOAD_GLOBAL              0 (x)
             22 LOAD_GLOBAL              0 (x)
             24 COMPARE_OP               9 (is not)
             26 LOAD_CONST               1 (('x',))
             28 BUILD_CONST_KEY_MAP      1
             30 LOAD_CONST               4 (<code object g at 0x7f3754f135f0, file "/home/pablogsal/github/python/master/lel.py", line 4>)
             32 LOAD_CONST               5 ('code.<locals>.g')
             34 MAKE_FUNCTION            4 (annotations)
             36 STORE_FAST               1 (g)
             38 LOAD_CONST               0 (None)
             40 RETURN_VALUE

Adding the missing line makes them do the same.

Thanks again for the fix and the investigation!