Ken Jin <kenjin4...@gmail.com> added the comment:

Wow! Thanks, that's an interesting find. My hunch is that we should be passing 
in _typevar_types and _paramspec_tvars in the copy_with of _GenericAlias and 
_ConcatenateGenericAlias. Inconsistent copy_with could trigger subtle bugs in 
ForwardRefs and get_type_hints. I can reproduce a semi-bug right now:

>>> P = ParamSpec('P')
>>> Callable[P, int].__parameters__
(~P,)
>>> Callable[P, int].copy_with((P,int))
typing.Callable[~P, int]
>>> Callable[P, int].copy_with((P,int)).__parameters__
()
^ This shouldn't be empty!!

Would you like to submit a patch for this?

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<https://bugs.python.org/issue46581>
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