some error messages in argument unpacking are different from CPython

[slozier]

The following function calls both raise a SyntaxError:

def f(**kwargs): pass

f(**{'x': 3}, **{'x': 5}, y=2)
f(x=5, **{'x': 3}, **{'x': 2})

with the error message:

TypeError: function got multiple values for keyword argument 'x'

However, with CPython 3.6 both should have the following error message:

TypeError: f() got multiple values for keyword argument 'x'

Note: The difference is minor, but I'm reporting it because the exact wording of the error messages is checked in test_unpack_ex.

[BCSharp]

A few random observations.

With #1173, we have now:

class C(**{'x': 3}, **{'x': 5}): pass

producing:

TypeError: function got multiple values for keyword argument 'x'

Not that CPython is perfect here either:

TypeError: __build_class__() got multiple values for keyword argument 'x'

Fixing it for class definitions would be easy, since the error appears at the definition place, where the name of the class is readily available. The message could be:

TypeError: definition of class 'C' got multiple values for keyword argument 'x'

Fixing it for functions would be more challenging, since it has to be done at the invocation place, meaning, the callable expression has to be inspected at runtime. For instace, CPython does:

>>> [](**{'x': 3}, **{'x': 5})   
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list object got multiple values for keyword argument 'x'
>>> None(**{'x': 3}, **{'x': 5})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: NoneType object got multiple values for keyword argument 'x'

And the funniest:

>>> x = object()
>>> x(**{'x': 3}, **{'x': 5})  
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object object got multiple values for keyword argument 'x'

Reminds me of Mayor Mayor from Catch 22.