whatsnew: ChainMap.new_child *m* argument.

And NEWS entry.

Doc/whatsnew/3.4.rst

@@ -596,6 +596,15 @@ including the Adobe variant.  (Contributed by Martin Morrison, the Mercurial
 project, Serhiy Storchaka, and Antoine Pitrou in :issue:`17618`.)
 
 
+collections
+-----------
+
+The :meth:`.ChainMap.new_child` method now accepts an *m* argument specifying
+the child map to add to the chain.  This allows an existing mapping and/or a
+custom mapping type to be used for the child.  (Contributed by Vinay Sajip in
+:issue:`16613`.)
+
+
 colorsys
 --------
 

Misc/NEWS

@@ -2376,6 +2376,9 @@ Core and Builtins
 - Issue #14850: Now a charmap decoder treats U+FFFE as "undefined mapping"
   in any mapping, not only in a string.
 
+- Issue #16613: Add *m* argument to ``collections.Chainmap.new_child`` to
+  allow the new child map to be specified explicitly.
+
 - Issue #16730: importlib.machinery.FileFinder now no longers raises an
   exception when trying to populate its cache and it finds out the directory is
   unreadable or has turned into a file. Reported and diagnosed by