Guido wrote:
I think it’s a reasonable idea and encourage you to start working on a design for the API and then a PRP. 

In this post, I explore how the new API might interact with dict objects. (I think PRP is a typo for PEP.)

 Here is an example of the present behaviour
    >>> d = dict()
    >>> d[x=1, y=2] = 3
    SyntaxError: invalid syntax
with the syntax error occurring at the first '=' symbol.

So what should be the new behaviour of:
    >>> d = dict()
    >>> d[x=1, y=2] = 3

To me, it is clear that we won't get a syntax error. This is because the proposal implies that
    >>> d[x=1, y=2] = 3
executes without error, for some values of 'd'.

If these two lines
    >>> d = dict()
    >>> d[x=1, y=2] = 3
execute without error, then I would the expect
    >>> len(d)
    >>> d[x=1, y=2]

I would further expect
    >>> k = list(d.keys())[0]
to give an object 'k', with the property
    >>> d[k]

This object 'k' corresponds to the source code fragment
    x=1, y=2
    d[x=1, y=2]
and I would expect 'k' to work as a key in any instance of dict.

Here is something that might help the reader. It is that current Python gives
    >>> d = dict()
    >>> d['x=1', 'y=2'] = 3
    >>> list(d.keys())[0]
    ('x=1', 'y=2')
and we are looking for an analogous object for
    >>> d[x=1, y=2] = 3

To summarize and slightly extend the above. If
    >>> d = dict()
    >>> d[x=1, y=2] = 3
executes without error, then there is an object 'k' such that
    >>> d[x=1, y=2] = 5
    >>> d[k] = 5
are equivalent. (This object 'k' is the key in the collection of (key, value) pairs that are the entries in the dict.)

Aside: The key object, if it exists, has a type. Whether this type is a new type or an existing type is an implementation question. This message is focussed on the user experience.

To conclude: If d[x=1, y=2] is allowed for 'd' a dict then consistency with the current dict behaviour requires the existence of a key object that corresponds to the fragment 'x=1, y=2'.

Finally, I have tried to be both clear and concise. I hope my contribution helps.