Incorrect `inspect.Signature` can be created: pos-only with a default followed by pos-or-kw without one

[sobolevn]

While working on #103553 I found one more problem that merits its own issue.

Right now it is impossible to create a function that have a pos-or-keyword parameter without a default comming after a pos-only parameter with a default. Demo:

>>> def test(pod=42, /, pk): ...
  File "<stdin>", line 1
    def test(pod=42, /, pk): ...
                        ^^
SyntaxError: parameter without a default follows parameter with a default

>>> lambda pod=42, /, pk: ...
  File "<stdin>", line 1
    lambda pod=42, /, pk: ...
                      ^^
SyntaxError: parameter without a default follows parameter with a default

But, it is possible to create a signature like this. Repro:

>>> import inspect
>>> def one(pk): ...
... 
>>> def two(pod=42, /): ...
... 
>>> sig1 = inspect.signature(one)
>>> sig2 = inspect.signature(two)
>>> inspect.Signature((sig2.parameters['pod'], sig1.parameters['pk']))
<Signature (pod=42, /, pk)>

This looks like a bug to me, there's no reason for a signature to behave differently and to support function signatures that cannot be created.

Desired behaviour:

Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/ex.py", line 7, in <module>
    print(inspect.Signature((sig2.parameters['pod'], sig1.parameters['pk'])))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython/Lib/inspect.py", line 3039, in __init__
    raise ValueError(msg)
ValueError: non-default argument follows default argument

The fix is quite easy. PR is incomming.

I've also tried the same thing with #103404 (because it is very related) but, it still does not solve this corner case.

Linked PRs

• gh-103556: [inspect.Signature] disallow pos-or-kw params without default after pos-only with default #103557
• [3.11] gh-103556: [inspect.Signature] disallow pos-or-kw params without default after pos-only with default (GH-103557) #103675