I saw that pablogsal welcomed improvments to the parser's suggestions, so here are the messages for parameters and arguments lists I think should be written instead of the current generic "invalid syntax".

>>> def foo(*arg, *arg): pass
SyntaxError: * argument may appear only once

>>> def foo(**arg, **arg): pass
SyntaxError: ** argument may appear only once

>>> def foo(arg1, /, arg2, /, arg3): pass
SyntaxError: / may appear only once

>>> def foo(*args, /, arg): pass
SyntaxError: / must be ahead of *

>>> def foo(/, arg): pass
SyntaxError: at least one argument must precede /

>>> def foo(arg=): pass
SyntaxError: expected default value expression

>>> def foo(*args=None): pass
SyntaxError: * argument cannot have default value

>>> def foo(**kwargs=None): pass
SyntaxError: ** argument cannot have default value

>>> foo(*args=[0])
SyntaxError: cannot assign to iterable argument unpacking

>>> foo(**args={"a": None})
SyntaxError: cannot assign to keyword argument unpacking

>>> foo(arg=)
SyntaxError: expected argument value expression

I also sometimes write def foo(pos_only, /*, kwarg): pass. Perhaps this can be special cased with 
SyntaxError: expected comma between / and *

Thanks a lot for the suggestions! I will try to give these a go to see if we can get them implemented. Parameter parsing is a bit hairy so not sure how lucky we will be but all of them make sense!

With two exceptions, nice suggestions if feasible.

>>> def foo(*args=None): pass
SyntaxError: * argument cannot have default value

>>> def foo(**kwargs=None): pass
SyntaxError: ** argument cannot have default value

Good.

>>> foo(*args=[0])
SyntaxError: cannot assign to iterable argument unpacking

>>> foo(**args={"a": None})
SyntaxError: cannot assign to keyword argument unpacking

Incomprehensible.  It seems to me that these should have same message as first two; message should not depend on proposed default.

@terry.reedy, I based that error message on current >>> foo(**{}, *())
SyntaxError: iterable argument unpacking follows keyword argument unpacking

and >>> foo(__debug__=True)
SyntaxError: cannot assign to __debug__

but the final error message could be anything if it explicitly says "what's wrong".

New changeset 7d810b6a4eab6eba689acc5bb05f85515478d690 by Pablo Galindo Salgado in branch 'main':
bpo-46838: Syntax error improvements for function definitions (GH-31590)
https://github.com/python/cpython/commit/7d810b6a4eab6eba689acc5bb05f85515478d690