Raising an error if buildout-versions or buildout.dumppickedversions is used

src/zc/buildout/buildout.py

@@ -960,6 +960,14 @@ class Buildout(DictMixin):
     def _load_extensions(self):
         __doing__ = 'Loading extensions.'
         specs = self['buildout'].get('extensions', '').split()
+        for superceded_extension in ['buildout-versions',
+                                     'buildout.dumppickedversions']:
+            if superceded_extension in specs:
+                msg = ("The extension %s is now included in buildout itself.\n"
+                       "Remove the extension from your configuration and "
+                       "look at the `show-picked-versions`\n"
+                       "option in buildout's documentation.")
+                raise zc.buildout.UserError(msg % superceded_extension)
         if specs:
             path = [self['buildout']['develop-eggs-directory']]
             if self.offline:

src/zc/buildout/repeatable.txt

@@ -370,6 +370,27 @@ And re-running buildout doesn't report any picked versions anymore:
     Updating foo.
     recipe v2
 
+Because buildout now includes buildout-versions' (and the older
+buildout.dumppickedversions') functionality, it warns if these extensions are
+configured.
+
+    >>> write(sample_buildout, 'buildout.cfg',
+    ... """
+    ... [buildout]
+    ... parts = foo
+    ... extensions = buildout-versions
+    ...
+    ... [foo]
+    ... recipe = spam
+    ... """)
+    >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
+    While:
+      Installing.
+      Loading extensions.
+    Error: The extension buildout-versions is now included in buildout itself.
+    Remove the extension from your configuration and look at the `show-picked-versions`
+    option in buildout's documentation.
+
 
 Controlling the python version
 -------------------------------