PEP 570: Demonstrate a corner case (#978)

The name of a positional-only parameter can occur in `**kwds`.

pep-0570.rst

@@ -514,6 +514,36 @@ and for ``varargslist`` would be::
       | '**' vfpdef [',']
     )
 
+--------------------
+Semantic Corner Case
+--------------------
+
+The following is an interesting corollary of the specification.
+Consider this function definition::
+
+    def foo(name, **kwds):
+        return 'name' in kwds
+
+There is no possible call that will make it return ``True``.
+For example::
+
+    >>> foo(1, **{'name': 2})
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    TypeError: foo() got multiple values for argument 'name'
+    >>>
+
+But using ``/`` we can support this::
+
+    def foo(name, /, **kwds):
+        return 'name' in kwds
+
+Now the above call will return ``True``.
+
+In other words, the names of positional-only parameters can be used in
+``**kwds`` without ambiguity.  (As another example, this benefits the
+signatures of ``dict()`` and ``dict.update()``.)
+
 ----------------------------
 Origin of "/" as a Separator
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