Clarify positional argument handling in TypedDict constructor

conformance/tests/typeddict_constructor_positional.py

@@ -0,0 +1,18 @@
+from typing import TypedDict
+
+class TD1(TypedDict):
+    a: int
+
+class TD2(TD1):
+    b: str
+
+# keyword arguments (OK)
+TD1(a=1)
+
+# positional TypedDict (OK)
+td2: TD2 = {"a": 1, "b": "x"}
+TD1(td2)
+
+# invalid positional arguments
+TD1({"a": 1})  # E
+TD1([("a", 1)])  # E

docs/spec/typeddict.rst

@@ -266,7 +266,15 @@ The TypedDict constructor
 
 TypedDict types are callable at runtime and can be used as a constructor
 to create values that conform to the TypedDict type. The constructor
-takes only keyword arguments, corresponding to the items of the TypedDict.
+primarily takes keyword arguments, corresponding to the items of the TypedDict.
+
+Additionally, type checkers may allow a call with a single positional argument
+whose type is a TypedDict. In this case, the resulting value is initialized from
+that argument.
+
+Other forms of positional arguments (such as arbitrary mappings or sequences)
+should not be accepted by type checkers.
+
 Example::
 
     m = Movie(name='Blade Runner', year=1982)