Substitution of ParamSpec in Concatenate produces weird results:

>>> import typing
>>> P = typing.ParamSpec('P')
>>> typing.Concatenate[str, P][int]
typing.Concatenate[str, int]
>>> typing.Concatenate[str, P][[int]]
typing.Concatenate[str, (<class 'int'>,)]
>>> typing.Concatenate[str, P][typing.Concatenate[int, P]]
typing.Concatenate[str, typing.Concatenate[int, ~P]]

But all these results are invalid:

>>> typing.Concatenate[str, int]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 309, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 400, in __getitem__
    return self._getitem(self, parameters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 595, in Concatenate
    raise TypeError("The last parameter to Concatenate should be a "
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: The last parameter to Concatenate should be a ParamSpec variable.
>>> typing.Concatenate[str, (int,)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 309, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 400, in __getitem__
    return self._getitem(self, parameters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 595, in Concatenate
    raise TypeError("The last parameter to Concatenate should be a "
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: The last parameter to Concatenate should be a ParamSpec variable.
>>> typing.Concatenate[str, [int]]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 309, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 400, in __getitem__
    return self._getitem(self, parameters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 595, in Concatenate
    raise TypeError("The last parameter to Concatenate should be a "
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: The last parameter to Concatenate should be a ParamSpec variable.
>>> typing.Concatenate[str, typing.Concatenate[int, P]]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 309, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 400, in __getitem__
    return self._getitem(self, parameters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/typing.py", line 595, in Concatenate
    raise TypeError("The last parameter to Concatenate should be a "
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: The last parameter to Concatenate should be a ParamSpec variable.

I expect that

1. The last parameter to Concatenate can be a Concatenate. Inner Concatenate should merge with the external one:

Concatenate[str, Concatenate[int, P]] -> Concatenate[str, int, P]
Concatenate[str, P][Concatenate[int, P]] -> Concatenate[str, int, P]

2. The last parameter to Concatenate can be a list of types. The result should be a list of types.

Concatenate[str, [int, dict]] -> [str, int, dict]
Concatenate[str, P][[int, dict]] -> [str, int, dict]

3. The last parameter to Concatenate can be an ellipsis.

Concatenate[str, ...]
Concatenate[str, P][...] -> Concatenate[str, ...]